Please welcome ZFS - The last word in file systems.

ZFS file system was ported from OpenSolaris operating system. The code in under
CDDL license.

I'd like to thank all SUN developers that created this great piece of software.

Supported by:	Wheel LTD (http://www.wheel.pl/)
Supported by:	The FreeBSD Foundation (http://www.freebsdfoundation.org/)
Supported by:	Sentex (http://www.sentex.net/)

1 files changed, 2829 insertions, 0 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
new file mode 100644
index 0000000..59a376a
--- /dev/null
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
@@ -0,0 +1,2829 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
+ * Use is subject to license terms.
+ */
+
+#pragma ident	"%Z%%M%	%I%	%E% SMI"
+
+/*
+ * DVA-based Adjustable Replacement Cache
+ *
+ * While much of the theory of operation used here is
+ * based on the self-tuning, low overhead replacement cache
+ * presented by Megiddo and Modha at FAST 2003, there are some
+ * significant differences:
+ *
+ * 1. The Megiddo and Modha model assumes any page is evictable.
+ * Pages in its cache cannot be "locked" into memory.  This makes
+ * the eviction algorithm simple: evict the last page in the list.
+ * This also make the performance characteristics easy to reason
+ * about.  Our cache is not so simple.  At any given moment, some
+ * subset of the blocks in the cache are un-evictable because we
+ * have handed out a reference to them.  Blocks are only evictable
+ * when there are no external references active.  This makes
+ * eviction far more problematic:  we choose to evict the evictable
+ * blocks that are the "lowest" in the list.
+ *
+ * There are times when it is not possible to evict the requested
+ * space.  In these circumstances we are unable to adjust the cache
+ * size.  To prevent the cache growing unbounded at these times we
+ * implement a "cache throttle" that slowes the flow of new data
+ * into the cache until we can make space avaiable.
+ *
+ * 2. The Megiddo and Modha model assumes a fixed cache size.
+ * Pages are evicted when the cache is full and there is a cache
+ * miss.  Our model has a variable sized cache.  It grows with
+ * high use, but also tries to react to memory preasure from the
+ * operating system: decreasing its size when system memory is
+ * tight.
+ *
+ * 3. The Megiddo and Modha model assumes a fixed page size. All
+ * elements of the cache are therefor exactly the same size.  So
+ * when adjusting the cache size following a cache miss, its simply
+ * a matter of choosing a single page to evict.  In our model, we
+ * have variable sized cache blocks (rangeing from 512 bytes to
+ * 128K bytes).  We therefor choose a set of blocks to evict to make
+ * space for a cache miss that approximates as closely as possible
+ * the space used by the new block.
+ *
+ * See also:  "ARC: A Self-Tuning, Low Overhead Replacement Cache"
+ * by N. Megiddo & D. Modha, FAST 2003
+ */
+
+/*
+ * The locking model:
+ *
+ * A new reference to a cache buffer can be obtained in two
+ * ways: 1) via a hash table lookup using the DVA as a key,
+ * or 2) via one of the ARC lists.  The arc_read() inerface
+ * uses method 1, while the internal arc algorithms for
+ * adjusting the cache use method 2.  We therefor provide two
+ * types of locks: 1) the hash table lock array, and 2) the
+ * arc list locks.
+ *
+ * Buffers do not have their own mutexs, rather they rely on the
+ * hash table mutexs for the bulk of their protection (i.e. most
+ * fields in the arc_buf_hdr_t are protected by these mutexs).
+ *
+ * buf_hash_find() returns the appropriate mutex (held) when it
+ * locates the requested buffer in the hash table.  It returns
+ * NULL for the mutex if the buffer was not in the table.
+ *
+ * buf_hash_remove() expects the appropriate hash mutex to be
+ * already held before it is invoked.
+ *
+ * Each arc state also has a mutex which is used to protect the
+ * buffer list associated with the state.  When attempting to
+ * obtain a hash table lock while holding an arc list lock you
+ * must use: mutex_tryenter() to avoid deadlock.  Also note that
+ * the active state mutex must be held before the ghost state mutex.
+ *
+ * Arc buffers may have an associated eviction callback function.
+ * This function will be invoked prior to removing the buffer (e.g.
+ * in arc_do_user_evicts()).  Note however that the data associated
+ * with the buffer may be evicted prior to the callback.  The callback
+ * must be made with *no locks held* (to prevent deadlock).  Additionally,
+ * the users of callbacks must ensure that their private data is
+ * protected from simultaneous callbacks from arc_buf_evict()
+ * and arc_do_user_evicts().
+ *
+ * Note that the majority of the performance stats are manipulated
+ * with atomic operations.
+ */
+
+#include <sys/spa.h>
+#include <sys/zio.h>
+#include <sys/zio_checksum.h>
+#include <sys/zfs_context.h>
+#include <sys/arc.h>
+#include <sys/refcount.h>
+#ifdef _KERNEL
+#include <sys/dnlc.h>
+#endif
+#include <sys/callb.h>
+#include <sys/kstat.h>
+#include <sys/sdt.h>
+
+#define	ARC_FREE_AT_ONCE	4194304
+
+static kmutex_t		arc_reclaim_thr_lock;
+static kcondvar_t	arc_reclaim_thr_cv;	/* used to signal reclaim thr */
+static uint8_t		arc_thread_exit;
+
+#define	ARC_REDUCE_DNLC_PERCENT	3
+uint_t arc_reduce_dnlc_percent = ARC_REDUCE_DNLC_PERCENT;
+
+typedef enum arc_reclaim_strategy {
+	ARC_RECLAIM_AGGR,		/* Aggressive reclaim strategy */
+	ARC_RECLAIM_CONS		/* Conservative reclaim strategy */
+} arc_reclaim_strategy_t;
+
+/* number of seconds before growing cache again */
+static int		arc_grow_retry = 60;
+
+/*
+ * minimum lifespan of a prefetch block in clock ticks
+ * (initialized in arc_init())
+ */
+static int		arc_min_prefetch_lifespan;
+
+static int arc_dead;
+
+/*
+ * These tunables are for performance analysis.
+ */
+uint64_t zfs_arc_max;
+uint64_t zfs_arc_min;
+
+/*
+ * Note that buffers can be on one of 5 states:
+ *	ARC_anon	- anonymous (discussed below)
+ *	ARC_mru		- recently used, currently cached
+ *	ARC_mru_ghost	- recentely used, no longer in cache
+ *	ARC_mfu		- frequently used, currently cached
+ *	ARC_mfu_ghost	- frequently used, no longer in cache
+ * When there are no active references to the buffer, they
+ * are linked onto one of the lists in arc.  These are the
+ * only buffers that can be evicted or deleted.
+ *
+ * Anonymous buffers are buffers that are not associated with
+ * a DVA.  These are buffers that hold dirty block copies
+ * before they are written to stable storage.  By definition,
+ * they are "ref'd" and are considered part of arc_mru
+ * that cannot be freed.  Generally, they will aquire a DVA
+ * as they are written and migrate onto the arc_mru list.
+ */
+
+typedef struct arc_state {
+	list_t	arcs_list;	/* linked list of evictable buffer in state */
+	uint64_t arcs_lsize;	/* total size of buffers in the linked list */
+	uint64_t arcs_size;	/* total size of all buffers in this state */
+	kmutex_t arcs_mtx;
+} arc_state_t;
+
+/* The 5 states: */
+static arc_state_t ARC_anon;
+static arc_state_t ARC_mru;
+static arc_state_t ARC_mru_ghost;
+static arc_state_t ARC_mfu;
+static arc_state_t ARC_mfu_ghost;
+
+typedef struct arc_stats {
+	kstat_named_t arcstat_hits;
+	kstat_named_t arcstat_misses;
+	kstat_named_t arcstat_demand_data_hits;
+	kstat_named_t arcstat_demand_data_misses;
+	kstat_named_t arcstat_demand_metadata_hits;
+	kstat_named_t arcstat_demand_metadata_misses;
+	kstat_named_t arcstat_prefetch_data_hits;
+	kstat_named_t arcstat_prefetch_data_misses;
+	kstat_named_t arcstat_prefetch_metadata_hits;
+	kstat_named_t arcstat_prefetch_metadata_misses;
+	kstat_named_t arcstat_mru_hits;
+	kstat_named_t arcstat_mru_ghost_hits;
+	kstat_named_t arcstat_mfu_hits;
+	kstat_named_t arcstat_mfu_ghost_hits;
+	kstat_named_t arcstat_deleted;
+	kstat_named_t arcstat_recycle_miss;
+	kstat_named_t arcstat_mutex_miss;
+	kstat_named_t arcstat_evict_skip;
+	kstat_named_t arcstat_hash_elements;
+	kstat_named_t arcstat_hash_elements_max;
+	kstat_named_t arcstat_hash_collisions;
+	kstat_named_t arcstat_hash_chains;
+	kstat_named_t arcstat_hash_chain_max;
+	kstat_named_t arcstat_p;
+	kstat_named_t arcstat_c;
+	kstat_named_t arcstat_c_min;
+	kstat_named_t arcstat_c_max;
+	kstat_named_t arcstat_size;
+} arc_stats_t;
+
+static arc_stats_t arc_stats = {
+	{ "hits",			KSTAT_DATA_UINT64 },
+	{ "misses",			KSTAT_DATA_UINT64 },
+	{ "demand_data_hits",		KSTAT_DATA_UINT64 },
+	{ "demand_data_misses",		KSTAT_DATA_UINT64 },
+	{ "demand_metadata_hits",	KSTAT_DATA_UINT64 },
+	{ "demand_metadata_misses",	KSTAT_DATA_UINT64 },
+	{ "prefetch_data_hits",		KSTAT_DATA_UINT64 },
+	{ "prefetch_data_misses",	KSTAT_DATA_UINT64 },
+	{ "prefetch_metadata_hits",	KSTAT_DATA_UINT64 },
+	{ "prefetch_metadata_misses",	KSTAT_DATA_UINT64 },
+	{ "mru_hits",			KSTAT_DATA_UINT64 },
+	{ "mru_ghost_hits",		KSTAT_DATA_UINT64 },
+	{ "mfu_hits",			KSTAT_DATA_UINT64 },
+	{ "mfu_ghost_hits",		KSTAT_DATA_UINT64 },
+	{ "deleted",			KSTAT_DATA_UINT64 },
+	{ "recycle_miss",		KSTAT_DATA_UINT64 },
+	{ "mutex_miss",			KSTAT_DATA_UINT64 },
+	{ "evict_skip",			KSTAT_DATA_UINT64 },
+	{ "hash_elements",		KSTAT_DATA_UINT64 },
+	{ "hash_elements_max",		KSTAT_DATA_UINT64 },
+	{ "hash_collisions",		KSTAT_DATA_UINT64 },
+	{ "hash_chains",		KSTAT_DATA_UINT64 },
+	{ "hash_chain_max",		KSTAT_DATA_UINT64 },
+	{ "p",				KSTAT_DATA_UINT64 },
+	{ "c",				KSTAT_DATA_UINT64 },
+	{ "c_min",			KSTAT_DATA_UINT64 },
+	{ "c_max",			KSTAT_DATA_UINT64 },
+	{ "size",			KSTAT_DATA_UINT64 }
+};
+
+#define	ARCSTAT(stat)	(arc_stats.stat.value.ui64)
+
+#define	ARCSTAT_INCR(stat, val) \
+	atomic_add_64(&arc_stats.stat.value.ui64, (val));
+
+#define	ARCSTAT_BUMP(stat) 	ARCSTAT_INCR(stat, 1)
+#define	ARCSTAT_BUMPDOWN(stat)	ARCSTAT_INCR(stat, -1)
+
+#define	ARCSTAT_MAX(stat, val) {					\
+	uint64_t m;							\
+	while ((val) > (m = arc_stats.stat.value.ui64) &&		\
+	    (m != atomic_cas_64(&arc_stats.stat.value.ui64, m, (val))))	\
+		continue;						\
+}
+
+#define	ARCSTAT_MAXSTAT(stat) \
+	ARCSTAT_MAX(stat##_max, arc_stats.stat.value.ui64)
+
+/*
+ * We define a macro to allow ARC hits/misses to be easily broken down by
+ * two separate conditions, giving a total of four different subtypes for
+ * each of hits and misses (so eight statistics total).
+ */
+#define	ARCSTAT_CONDSTAT(cond1, stat1, notstat1, cond2, stat2, notstat2, stat) \
+	if (cond1) {							\
+		if (cond2) {						\
+			ARCSTAT_BUMP(arcstat_##stat1##_##stat2##_##stat); \
+		} else {						\
+			ARCSTAT_BUMP(arcstat_##stat1##_##notstat2##_##stat); \
+		}							\
+	} else {							\
+		if (cond2) {						\
+			ARCSTAT_BUMP(arcstat_##notstat1##_##stat2##_##stat); \
+		} else {						\
+			ARCSTAT_BUMP(arcstat_##notstat1##_##notstat2##_##stat);\
+		}							\
+	}
+
+kstat_t			*arc_ksp;
+static arc_state_t 	*arc_anon;
+static arc_state_t	*arc_mru;
+static arc_state_t	*arc_mru_ghost;
+static arc_state_t	*arc_mfu;
+static arc_state_t	*arc_mfu_ghost;
+
+/*
+ * There are several ARC variables that are critical to export as kstats --
+ * but we don't want to have to grovel around in the kstat whenever we wish to
+ * manipulate them.  For these variables, we therefore define them to be in
+ * terms of the statistic variable.  This assures that we are not introducing
+ * the possibility of inconsistency by having shadow copies of the variables,
+ * while still allowing the code to be readable.
+ */
+#define	arc_size	ARCSTAT(arcstat_size)	/* actual total arc size */
+#define	arc_p		ARCSTAT(arcstat_p)	/* target size of MRU */
+#define	arc_c		ARCSTAT(arcstat_c)	/* target size of cache */
+#define	arc_c_min	ARCSTAT(arcstat_c_min)	/* min target cache size */
+#define	arc_c_max	ARCSTAT(arcstat_c_max)	/* max target cache size */
+
+static int		arc_no_grow;	/* Don't try to grow cache size */
+static uint64_t		arc_tempreserve;
+
+typedef struct arc_callback arc_callback_t;
+
+struct arc_callback {
+	void			*acb_private;
+	arc_done_func_t		*acb_done;
+	arc_byteswap_func_t	*acb_byteswap;
+	arc_buf_t		*acb_buf;
+	zio_t			*acb_zio_dummy;
+	arc_callback_t		*acb_next;
+};
+
+typedef struct arc_write_callback arc_write_callback_t;
+
+struct arc_write_callback {
+	void		*awcb_private;
+	arc_done_func_t	*awcb_ready;
+	arc_done_func_t	*awcb_done;
+	arc_buf_t	*awcb_buf;
+};
+
+struct arc_buf_hdr {
+	/* protected by hash lock */
+	dva_t			b_dva;
+	uint64_t		b_birth;
+	uint64_t		b_cksum0;
+
+	kmutex_t		b_freeze_lock;
+	zio_cksum_t		*b_freeze_cksum;
+
+	arc_buf_hdr_t		*b_hash_next;
+	arc_buf_t		*b_buf;
+	uint32_t		b_flags;
+	uint32_t		b_datacnt;
+
+	arc_callback_t		*b_acb;
+	kcondvar_t		b_cv;
+
+	/* immutable */
+	arc_buf_contents_t	b_type;
+	uint64_t		b_size;
+	spa_t			*b_spa;
+
+	/* protected by arc state mutex */
+	arc_state_t		*b_state;
+	list_node_t		b_arc_node;
+
+	/* updated atomically */
+	clock_t			b_arc_access;
+
+	/* self protecting */
+	refcount_t		b_refcnt;
+};
+
+static arc_buf_t *arc_eviction_list;
+static kmutex_t arc_eviction_mtx;
+static arc_buf_hdr_t arc_eviction_hdr;
+static void arc_get_data_buf(arc_buf_t *buf);
+static void arc_access(arc_buf_hdr_t *buf, kmutex_t *hash_lock);
+
+#define	GHOST_STATE(state)	\
+	((state) == arc_mru_ghost || (state) == arc_mfu_ghost)
+
+/*
+ * Private ARC flags.  These flags are private ARC only flags that will show up
+ * in b_flags in the arc_hdr_buf_t.  Some flags are publicly declared, and can
+ * be passed in as arc_flags in things like arc_read.  However, these flags
+ * should never be passed and should only be set by ARC code.  When adding new
+ * public flags, make sure not to smash the private ones.
+ */
+
+#define	ARC_IN_HASH_TABLE	(1 << 9)	/* this buffer is hashed */
+#define	ARC_IO_IN_PROGRESS	(1 << 10)	/* I/O in progress for buf */
+#define	ARC_IO_ERROR		(1 << 11)	/* I/O failed for buf */
+#define	ARC_FREED_IN_READ	(1 << 12)	/* buf freed while in read */
+#define	ARC_BUF_AVAILABLE	(1 << 13)	/* block not in active use */
+#define	ARC_INDIRECT		(1 << 14)	/* this is an indirect block */
+
+#define	HDR_IN_HASH_TABLE(hdr)	((hdr)->b_flags & ARC_IN_HASH_TABLE)
+#define	HDR_IO_IN_PROGRESS(hdr)	((hdr)->b_flags & ARC_IO_IN_PROGRESS)
+#define	HDR_IO_ERROR(hdr)	((hdr)->b_flags & ARC_IO_ERROR)
+#define	HDR_FREED_IN_READ(hdr)	((hdr)->b_flags & ARC_FREED_IN_READ)
+#define	HDR_BUF_AVAILABLE(hdr)	((hdr)->b_flags & ARC_BUF_AVAILABLE)
+
+/*
+ * Hash table routines
+ */
+
+#define	HT_LOCK_PAD	128
+
+struct ht_lock {
+	kmutex_t	ht_lock;
+#ifdef _KERNEL
+	unsigned char	pad[(HT_LOCK_PAD - sizeof (kmutex_t))];
+#endif
+};
+
+#define	BUF_LOCKS 256
+typedef struct buf_hash_table {
+	uint64_t ht_mask;
+	arc_buf_hdr_t **ht_table;
+	struct ht_lock ht_locks[BUF_LOCKS];
+} buf_hash_table_t;
+
+static buf_hash_table_t buf_hash_table;
+
+#define	BUF_HASH_INDEX(spa, dva, birth) \
+	(buf_hash(spa, dva, birth) & buf_hash_table.ht_mask)
+#define	BUF_HASH_LOCK_NTRY(idx) (buf_hash_table.ht_locks[idx & (BUF_LOCKS-1)])
+#define	BUF_HASH_LOCK(idx)	(&(BUF_HASH_LOCK_NTRY(idx).ht_lock))
+#define	HDR_LOCK(buf) \
+	(BUF_HASH_LOCK(BUF_HASH_INDEX(buf->b_spa, &buf->b_dva, buf->b_birth)))
+
+uint64_t zfs_crc64_table[256];
+
+static uint64_t
+buf_hash(spa_t *spa, dva_t *dva, uint64_t birth)
+{
+	uintptr_t spav = (uintptr_t)spa;
+	uint8_t *vdva = (uint8_t *)dva;
+	uint64_t crc = -1ULL;
+	int i;
+
+	ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY);
+
+	for (i = 0; i < sizeof (dva_t); i++)
+		crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ vdva[i]) & 0xFF];
+
+	crc ^= (spav>>8) ^ birth;
+
+	return (crc);
+}
+
+#define	BUF_EMPTY(buf)						\
+	((buf)->b_dva.dva_word[0] == 0 &&			\
+	(buf)->b_dva.dva_word[1] == 0 &&			\
+	(buf)->b_birth == 0)
+
+#define	BUF_EQUAL(spa, dva, birth, buf)				\
+	((buf)->b_dva.dva_word[0] == (dva)->dva_word[0]) &&	\
+	((buf)->b_dva.dva_word[1] == (dva)->dva_word[1]) &&	\
+	((buf)->b_birth == birth) && ((buf)->b_spa == spa)
+
+static arc_buf_hdr_t *
+buf_hash_find(spa_t *spa, dva_t *dva, uint64_t birth, kmutex_t **lockp)
+{
+	uint64_t idx = BUF_HASH_INDEX(spa, dva, birth);
+	kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
+	arc_buf_hdr_t *buf;
+
+	mutex_enter(hash_lock);
+	for (buf = buf_hash_table.ht_table[idx]; buf != NULL;
+	    buf = buf->b_hash_next) {
+		if (BUF_EQUAL(spa, dva, birth, buf)) {
+			*lockp = hash_lock;
+			return (buf);
+		}
+	}
+	mutex_exit(hash_lock);
+	*lockp = NULL;
+	return (NULL);
+}
+
+/*
+ * Insert an entry into the hash table.  If there is already an element
+ * equal to elem in the hash table, then the already existing element
+ * will be returned and the new element will not be inserted.
+ * Otherwise returns NULL.
+ */
+static arc_buf_hdr_t *
+buf_hash_insert(arc_buf_hdr_t *buf, kmutex_t **lockp)
+{
+	uint64_t idx = BUF_HASH_INDEX(buf->b_spa, &buf->b_dva, buf->b_birth);
+	kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
+	arc_buf_hdr_t *fbuf;
+	uint32_t i;
+
+	ASSERT(!HDR_IN_HASH_TABLE(buf));
+	*lockp = hash_lock;
+	mutex_enter(hash_lock);
+	for (fbuf = buf_hash_table.ht_table[idx], i = 0; fbuf != NULL;
+	    fbuf = fbuf->b_hash_next, i++) {
+		if (BUF_EQUAL(buf->b_spa, &buf->b_dva, buf->b_birth, fbuf))
+			return (fbuf);
+	}
+
+	buf->b_hash_next = buf_hash_table.ht_table[idx];
+	buf_hash_table.ht_table[idx] = buf;
+	buf->b_flags |= ARC_IN_HASH_TABLE;
+
+	/* collect some hash table performance data */
+	if (i > 0) {
+		ARCSTAT_BUMP(arcstat_hash_collisions);
+		if (i == 1)
+			ARCSTAT_BUMP(arcstat_hash_chains);
+
+		ARCSTAT_MAX(arcstat_hash_chain_max, i);
+	}
+
+	ARCSTAT_BUMP(arcstat_hash_elements);
+	ARCSTAT_MAXSTAT(arcstat_hash_elements);
+
+	return (NULL);
+}
+
+static void
+buf_hash_remove(arc_buf_hdr_t *buf)
+{
+	arc_buf_hdr_t *fbuf, **bufp;
+	uint64_t idx = BUF_HASH_INDEX(buf->b_spa, &buf->b_dva, buf->b_birth);
+
+	ASSERT(MUTEX_HELD(BUF_HASH_LOCK(idx)));
+	ASSERT(HDR_IN_HASH_TABLE(buf));
+
+	bufp = &buf_hash_table.ht_table[idx];
+	while ((fbuf = *bufp) != buf) {
+		ASSERT(fbuf != NULL);
+		bufp = &fbuf->b_hash_next;
+	}
+	*bufp = buf->b_hash_next;
+	buf->b_hash_next = NULL;
+	buf->b_flags &= ~ARC_IN_HASH_TABLE;
+
+	/* collect some hash table performance data */
+	ARCSTAT_BUMPDOWN(arcstat_hash_elements);
+
+	if (buf_hash_table.ht_table[idx] &&
+	    buf_hash_table.ht_table[idx]->b_hash_next == NULL)
+		ARCSTAT_BUMPDOWN(arcstat_hash_chains);
+}
+
+/*
+ * Global data structures and functions for the buf kmem cache.
+ */
+static kmem_cache_t *hdr_cache;
+static kmem_cache_t *buf_cache;
+
+static void
+buf_fini(void)
+{
+	int i;
+
+	kmem_free(buf_hash_table.ht_table,
+	    (buf_hash_table.ht_mask + 1) * sizeof (void *));
+	for (i = 0; i < BUF_LOCKS; i++)
+		mutex_destroy(&buf_hash_table.ht_locks[i].ht_lock);
+	kmem_cache_destroy(hdr_cache);
+	kmem_cache_destroy(buf_cache);
+}
+
+/*
+ * Constructor callback - called when the cache is empty
+ * and a new buf is requested.
+ */
+/* ARGSUSED */
+static int
+hdr_cons(void *vbuf, void *unused, int kmflag)
+{
+	arc_buf_hdr_t *buf = vbuf;
+
+	bzero(buf, sizeof (arc_buf_hdr_t));
+	refcount_create(&buf->b_refcnt);
+	cv_init(&buf->b_cv, NULL, CV_DEFAULT, NULL);
+	return (0);
+}
+
+/*
+ * Destructor callback - called when a cached buf is
+ * no longer required.
+ */
+/* ARGSUSED */
+static void
+hdr_dest(void *vbuf, void *unused)
+{
+	arc_buf_hdr_t *buf = vbuf;
+
+	refcount_destroy(&buf->b_refcnt);
+	cv_destroy(&buf->b_cv);
+}
+
+/*
+ * Reclaim callback -- invoked when memory is low.
+ */
+/* ARGSUSED */
+static void
+hdr_recl(void *unused)
+{
+	dprintf("hdr_recl called\n");
+	/*
+	 * umem calls the reclaim func when we destroy the buf cache,
+	 * which is after we do arc_fini().
+	 */
+	if (!arc_dead)
+		cv_signal(&arc_reclaim_thr_cv);
+}
+
+static void
+buf_init(void)
+{
+	uint64_t *ct;
+	uint64_t hsize = 1ULL << 12;
+	int i, j;
+
+	/*
+	 * The hash table is big enough to fill all of physical memory
+	 * with an average 64K block size.  The table will take up
+	 * totalmem*sizeof(void*)/64K (eg. 128KB/GB with 8-byte pointers).
+	 */
+	while (hsize * 65536 < physmem * PAGESIZE)
+		hsize <<= 1;
+retry:
+	buf_hash_table.ht_mask = hsize - 1;
+	buf_hash_table.ht_table =
+	    kmem_zalloc(hsize * sizeof (void*), KM_NOSLEEP);
+	if (buf_hash_table.ht_table == NULL) {
+		ASSERT(hsize > (1ULL << 8));
+		hsize >>= 1;
+		goto retry;
+	}
+
+	hdr_cache = kmem_cache_create("arc_buf_hdr_t", sizeof (arc_buf_hdr_t),
+	    0, hdr_cons, hdr_dest, hdr_recl, NULL, NULL, 0);
+	buf_cache = kmem_cache_create("arc_buf_t", sizeof (arc_buf_t),
+	    0, NULL, NULL, NULL, NULL, NULL, 0);
+
+	for (i = 0; i < 256; i++)
+		for (ct = zfs_crc64_table + i, *ct = i, j = 8; j > 0; j--)
+			*ct = (*ct >> 1) ^ (-(*ct & 1) & ZFS_CRC64_POLY);
+
+	for (i = 0; i < BUF_LOCKS; i++) {
+		mutex_init(&buf_hash_table.ht_locks[i].ht_lock,
+		    NULL, MUTEX_DEFAULT, NULL);
+	}
+}
+
+#define	ARC_MINTIME	(hz>>4) /* 62 ms */
+
+static void
+arc_cksum_verify(arc_buf_t *buf)
+{
+	zio_cksum_t zc;
+
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	mutex_enter(&buf->b_hdr->b_freeze_lock);
+	if (buf->b_hdr->b_freeze_cksum == NULL ||
+	    (buf->b_hdr->b_flags & ARC_IO_ERROR)) {
+		mutex_exit(&buf->b_hdr->b_freeze_lock);
+		return;
+	}
+	fletcher_2_native(buf->b_data, buf->b_hdr->b_size, &zc);
+	if (!ZIO_CHECKSUM_EQUAL(*buf->b_hdr->b_freeze_cksum, zc))
+		panic("buffer modified while frozen!");
+	mutex_exit(&buf->b_hdr->b_freeze_lock);
+}
+
+static void
+arc_cksum_compute(arc_buf_t *buf)
+{
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	mutex_enter(&buf->b_hdr->b_freeze_lock);
+	if (buf->b_hdr->b_freeze_cksum != NULL) {
+		mutex_exit(&buf->b_hdr->b_freeze_lock);
+		return;
+	}
+	buf->b_hdr->b_freeze_cksum = kmem_alloc(sizeof (zio_cksum_t), KM_SLEEP);
+	fletcher_2_native(buf->b_data, buf->b_hdr->b_size,
+	    buf->b_hdr->b_freeze_cksum);
+	mutex_exit(&buf->b_hdr->b_freeze_lock);
+}
+
+void
+arc_buf_thaw(arc_buf_t *buf)
+{
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	if (buf->b_hdr->b_state != arc_anon)
+		panic("modifying non-anon buffer!");
+	if (buf->b_hdr->b_flags & ARC_IO_IN_PROGRESS)
+		panic("modifying buffer while i/o in progress!");
+	arc_cksum_verify(buf);
+	mutex_enter(&buf->b_hdr->b_freeze_lock);
+	if (buf->b_hdr->b_freeze_cksum != NULL) {
+		kmem_free(buf->b_hdr->b_freeze_cksum, sizeof (zio_cksum_t));
+		buf->b_hdr->b_freeze_cksum = NULL;
+	}
+	mutex_exit(&buf->b_hdr->b_freeze_lock);
+}
+
+void
+arc_buf_freeze(arc_buf_t *buf)
+{
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	ASSERT(buf->b_hdr->b_freeze_cksum != NULL ||
+	    buf->b_hdr->b_state == arc_anon);
+	arc_cksum_compute(buf);
+}
+
+static void
+add_reference(arc_buf_hdr_t *ab, kmutex_t *hash_lock, void *tag)
+{
+	ASSERT(MUTEX_HELD(hash_lock));
+
+	if ((refcount_add(&ab->b_refcnt, tag) == 1) &&
+	    (ab->b_state != arc_anon)) {
+		uint64_t delta = ab->b_size * ab->b_datacnt;
+
+		ASSERT(!MUTEX_HELD(&ab->b_state->arcs_mtx));
+		mutex_enter(&ab->b_state->arcs_mtx);
+		ASSERT(list_link_active(&ab->b_arc_node));
+		list_remove(&ab->b_state->arcs_list, ab);
+		if (GHOST_STATE(ab->b_state)) {
+			ASSERT3U(ab->b_datacnt, ==, 0);
+			ASSERT3P(ab->b_buf, ==, NULL);
+			delta = ab->b_size;
+		}
+		ASSERT(delta > 0);
+		ASSERT3U(ab->b_state->arcs_lsize, >=, delta);
+		atomic_add_64(&ab->b_state->arcs_lsize, -delta);
+		mutex_exit(&ab->b_state->arcs_mtx);
+		/* remove the prefetch flag is we get a reference */
+		if (ab->b_flags & ARC_PREFETCH)
+			ab->b_flags &= ~ARC_PREFETCH;
+	}
+}
+
+static int
+remove_reference(arc_buf_hdr_t *ab, kmutex_t *hash_lock, void *tag)
+{
+	int cnt;
+	arc_state_t *state = ab->b_state;
+
+	ASSERT(state == arc_anon || MUTEX_HELD(hash_lock));
+	ASSERT(!GHOST_STATE(state));
+
+	if (((cnt = refcount_remove(&ab->b_refcnt, tag)) == 0) &&
+	    (state != arc_anon)) {
+		ASSERT(!MUTEX_HELD(&state->arcs_mtx));
+		mutex_enter(&state->arcs_mtx);
+		ASSERT(!list_link_active(&ab->b_arc_node));
+		list_insert_head(&state->arcs_list, ab);
+		ASSERT(ab->b_datacnt > 0);
+		atomic_add_64(&state->arcs_lsize, ab->b_size * ab->b_datacnt);
+		ASSERT3U(state->arcs_size, >=, state->arcs_lsize);
+		mutex_exit(&state->arcs_mtx);
+	}
+	return (cnt);
+}
+
+/*
+ * Move the supplied buffer to the indicated state.  The mutex
+ * for the buffer must be held by the caller.
+ */
+static void
+arc_change_state(arc_state_t *new_state, arc_buf_hdr_t *ab, kmutex_t *hash_lock)
+{
+	arc_state_t *old_state = ab->b_state;
+	int64_t refcnt = refcount_count(&ab->b_refcnt);
+	uint64_t from_delta, to_delta;
+
+	ASSERT(MUTEX_HELD(hash_lock));
+	ASSERT(new_state != old_state);
+	ASSERT(refcnt == 0 || ab->b_datacnt > 0);
+	ASSERT(ab->b_datacnt == 0 || !GHOST_STATE(new_state));
+
+	from_delta = to_delta = ab->b_datacnt * ab->b_size;
+
+	/*
+	 * If this buffer is evictable, transfer it from the
+	 * old state list to the new state list.
+	 */
+	if (refcnt == 0) {
+		if (old_state != arc_anon) {
+			int use_mutex = !MUTEX_HELD(&old_state->arcs_mtx);
+
+			if (use_mutex)
+				mutex_enter(&old_state->arcs_mtx);
+
+			ASSERT(list_link_active(&ab->b_arc_node));
+			list_remove(&old_state->arcs_list, ab);
+
+			/*
+			 * If prefetching out of the ghost cache,
+			 * we will have a non-null datacnt.
+			 */
+			if (GHOST_STATE(old_state) && ab->b_datacnt == 0) {
+				/* ghost elements have a ghost size */
+				ASSERT(ab->b_buf == NULL);
+				from_delta = ab->b_size;
+			}
+			ASSERT3U(old_state->arcs_lsize, >=, from_delta);
+			atomic_add_64(&old_state->arcs_lsize, -from_delta);
+
+			if (use_mutex)
+				mutex_exit(&old_state->arcs_mtx);
+		}
+		if (new_state != arc_anon) {
+			int use_mutex = !MUTEX_HELD(&new_state->arcs_mtx);
+
+			if (use_mutex)
+				mutex_enter(&new_state->arcs_mtx);
+
+			list_insert_head(&new_state->arcs_list, ab);
+
+			/* ghost elements have a ghost size */
+			if (GHOST_STATE(new_state)) {
+				ASSERT(ab->b_datacnt == 0);
+				ASSERT(ab->b_buf == NULL);
+				to_delta = ab->b_size;
+			}
+			atomic_add_64(&new_state->arcs_lsize, to_delta);
+			ASSERT3U(new_state->arcs_size + to_delta, >=,
+			    new_state->arcs_lsize);
+
+			if (use_mutex)
+				mutex_exit(&new_state->arcs_mtx);
+		}
+	}
+
+	ASSERT(!BUF_EMPTY(ab));
+	if (new_state == arc_anon && old_state != arc_anon) {
+		buf_hash_remove(ab);
+	}
+
+	/* adjust state sizes */
+	if (to_delta)
+		atomic_add_64(&new_state->arcs_size, to_delta);
+	if (from_delta) {
+		ASSERT3U(old_state->arcs_size, >=, from_delta);
+		atomic_add_64(&old_state->arcs_size, -from_delta);
+	}
+	ab->b_state = new_state;
+}
+
+arc_buf_t *
+arc_buf_alloc(spa_t *spa, int size, void *tag, arc_buf_contents_t type)
+{
+	arc_buf_hdr_t *hdr;
+	arc_buf_t *buf;
+
+	ASSERT3U(size, >, 0);
+	hdr = kmem_cache_alloc(hdr_cache, KM_SLEEP);
+	ASSERT(BUF_EMPTY(hdr));
+	hdr->b_size = size;
+	hdr->b_type = type;
+	hdr->b_spa = spa;
+	hdr->b_state = arc_anon;
+	hdr->b_arc_access = 0;
+	mutex_init(&hdr->b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
+	buf = kmem_cache_alloc(buf_cache, KM_SLEEP);
+	buf->b_hdr = hdr;
+	buf->b_data = NULL;
+	buf->b_efunc = NULL;
+	buf->b_private = NULL;
+	buf->b_next = NULL;
+	hdr->b_buf = buf;
+	arc_get_data_buf(buf);
+	hdr->b_datacnt = 1;
+	hdr->b_flags = 0;
+	ASSERT(refcount_is_zero(&hdr->b_refcnt));
+	(void) refcount_add(&hdr->b_refcnt, tag);
+
+	return (buf);
+}
+
+static arc_buf_t *
+arc_buf_clone(arc_buf_t *from)
+{
+	arc_buf_t *buf;
+	arc_buf_hdr_t *hdr = from->b_hdr;
+	uint64_t size = hdr->b_size;
+
+	buf = kmem_cache_alloc(buf_cache, KM_SLEEP);
+	buf->b_hdr = hdr;
+	buf->b_data = NULL;
+	buf->b_efunc = NULL;
+	buf->b_private = NULL;
+	buf->b_next = hdr->b_buf;
+	hdr->b_buf = buf;
+	arc_get_data_buf(buf);
+	bcopy(from->b_data, buf->b_data, size);
+	hdr->b_datacnt += 1;
+	return (buf);
+}
+
+void
+arc_buf_add_ref(arc_buf_t *buf, void* tag)
+{
+	arc_buf_hdr_t *hdr;
+	kmutex_t *hash_lock;
+
+	/*
+	 * Check to see if this buffer is currently being evicted via
+	 * arc_do_user_evicts().
+	 */
+	mutex_enter(&arc_eviction_mtx);
+	hdr = buf->b_hdr;
+	if (hdr == NULL) {
+		mutex_exit(&arc_eviction_mtx);
+		return;
+	}
+	hash_lock = HDR_LOCK(hdr);
+	mutex_exit(&arc_eviction_mtx);
+
+	mutex_enter(hash_lock);
+	if (buf->b_data == NULL) {
+		/*
+		 * This buffer is evicted.
+		 */
+		mutex_exit(hash_lock);
+		return;
+	}
+
+	ASSERT(buf->b_hdr == hdr);
+	ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);
+	add_reference(hdr, hash_lock, tag);
+	arc_access(hdr, hash_lock);
+	mutex_exit(hash_lock);
+	ARCSTAT_BUMP(arcstat_hits);
+	ARCSTAT_CONDSTAT(!(hdr->b_flags & ARC_PREFETCH),
+	    demand, prefetch, hdr->b_type != ARC_BUFC_METADATA,
+	    data, metadata, hits);
+}
+
+static void
+arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t all)
+{
+	arc_buf_t **bufp;
+
+	/* free up data associated with the buf */
+	if (buf->b_data) {
+		arc_state_t *state = buf->b_hdr->b_state;
+		uint64_t size = buf->b_hdr->b_size;
+		arc_buf_contents_t type = buf->b_hdr->b_type;
+
+		arc_cksum_verify(buf);
+		if (!recycle) {
+			if (type == ARC_BUFC_METADATA) {
+				zio_buf_free(buf->b_data, size);
+			} else {
+				ASSERT(type == ARC_BUFC_DATA);
+				zio_data_buf_free(buf->b_data, size);
+			}
+			atomic_add_64(&arc_size, -size);
+		}
+		if (list_link_active(&buf->b_hdr->b_arc_node)) {
+			ASSERT(refcount_is_zero(&buf->b_hdr->b_refcnt));
+			ASSERT(state != arc_anon);
+			ASSERT3U(state->arcs_lsize, >=, size);
+			atomic_add_64(&state->arcs_lsize, -size);
+		}
+		ASSERT3U(state->arcs_size, >=, size);
+		atomic_add_64(&state->arcs_size, -size);
+		buf->b_data = NULL;
+		ASSERT(buf->b_hdr->b_datacnt > 0);
+		buf->b_hdr->b_datacnt -= 1;
+	}
+
+	/* only remove the buf if requested */
+	if (!all)
+		return;
+
+	/* remove the buf from the hdr list */
+	for (bufp = &buf->b_hdr->b_buf; *bufp != buf; bufp = &(*bufp)->b_next)
+		continue;
+	*bufp = buf->b_next;
+
+	ASSERT(buf->b_efunc == NULL);
+
+	/* clean up the buf */
+	buf->b_hdr = NULL;
+	kmem_cache_free(buf_cache, buf);
+}
+
+static void
+arc_hdr_destroy(arc_buf_hdr_t *hdr)
+{
+	ASSERT(refcount_is_zero(&hdr->b_refcnt));
+	ASSERT3P(hdr->b_state, ==, arc_anon);
+	ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+
+	if (!BUF_EMPTY(hdr)) {
+		ASSERT(!HDR_IN_HASH_TABLE(hdr));
+		bzero(&hdr->b_dva, sizeof (dva_t));
+		hdr->b_birth = 0;
+		hdr->b_cksum0 = 0;
+	}
+	while (hdr->b_buf) {
+		arc_buf_t *buf = hdr->b_buf;
+
+		if (buf->b_efunc) {
+			mutex_enter(&arc_eviction_mtx);
+			ASSERT(buf->b_hdr != NULL);
+			arc_buf_destroy(hdr->b_buf, FALSE, FALSE);
+			hdr->b_buf = buf->b_next;
+			buf->b_hdr = &arc_eviction_hdr;
+			buf->b_next = arc_eviction_list;
+			arc_eviction_list = buf;
+			mutex_exit(&arc_eviction_mtx);
+		} else {
+			arc_buf_destroy(hdr->b_buf, FALSE, TRUE);
+		}
+	}
+	if (hdr->b_freeze_cksum != NULL) {
+		kmem_free(hdr->b_freeze_cksum, sizeof (zio_cksum_t));
+		hdr->b_freeze_cksum = NULL;
+	}
+	mutex_destroy(&hdr->b_freeze_lock);
+
+	ASSERT(!list_link_active(&hdr->b_arc_node));
+	ASSERT3P(hdr->b_hash_next, ==, NULL);
+	ASSERT3P(hdr->b_acb, ==, NULL);
+	kmem_cache_free(hdr_cache, hdr);
+}
+
+void
+arc_buf_free(arc_buf_t *buf, void *tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	int hashed = hdr->b_state != arc_anon;
+
+	ASSERT(buf->b_efunc == NULL);
+	ASSERT(buf->b_data != NULL);
+
+	if (hashed) {
+		kmutex_t *hash_lock = HDR_LOCK(hdr);
+
+		mutex_enter(hash_lock);
+		(void) remove_reference(hdr, hash_lock, tag);
+		if (hdr->b_datacnt > 1)
+			arc_buf_destroy(buf, FALSE, TRUE);
+		else
+			hdr->b_flags |= ARC_BUF_AVAILABLE;
+		mutex_exit(hash_lock);
+	} else if (HDR_IO_IN_PROGRESS(hdr)) {
+		int destroy_hdr;
+		/*
+		 * We are in the middle of an async write.  Don't destroy
+		 * this buffer unless the write completes before we finish
+		 * decrementing the reference count.
+		 */
+		mutex_enter(&arc_eviction_mtx);
+		(void) remove_reference(hdr, NULL, tag);
+		ASSERT(refcount_is_zero(&hdr->b_refcnt));
+		destroy_hdr = !HDR_IO_IN_PROGRESS(hdr);
+		mutex_exit(&arc_eviction_mtx);
+		if (destroy_hdr)
+			arc_hdr_destroy(hdr);
+	} else {
+		if (remove_reference(hdr, NULL, tag) > 0) {
+			ASSERT(HDR_IO_ERROR(hdr));
+			arc_buf_destroy(buf, FALSE, TRUE);
+		} else {
+			arc_hdr_destroy(hdr);
+		}
+	}
+}
+
+int
+arc_buf_remove_ref(arc_buf_t *buf, void* tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	kmutex_t *hash_lock = HDR_LOCK(hdr);
+	int no_callback = (buf->b_efunc == NULL);
+
+	if (hdr->b_state == arc_anon) {
+		arc_buf_free(buf, tag);
+		return (no_callback);
+	}
+
+	mutex_enter(hash_lock);
+	ASSERT(hdr->b_state != arc_anon);
+	ASSERT(buf->b_data != NULL);
+
+	(void) remove_reference(hdr, hash_lock, tag);
+	if (hdr->b_datacnt > 1) {
+		if (no_callback)
+			arc_buf_destroy(buf, FALSE, TRUE);
+	} else if (no_callback) {
+		ASSERT(hdr->b_buf == buf && buf->b_next == NULL);
+		hdr->b_flags |= ARC_BUF_AVAILABLE;
+	}
+	ASSERT(no_callback || hdr->b_datacnt > 1 ||
+	    refcount_is_zero(&hdr->b_refcnt));
+	mutex_exit(hash_lock);
+	return (no_callback);
+}
+
+int
+arc_buf_size(arc_buf_t *buf)
+{
+	return (buf->b_hdr->b_size);
+}
+
+/*
+ * Evict buffers from list until we've removed the specified number of
+ * bytes.  Move the removed buffers to the appropriate evict state.
+ * If the recycle flag is set, then attempt to "recycle" a buffer:
+ * - look for a buffer to evict that is `bytes' long.
+ * - return the data block from this buffer rather than freeing it.
+ * This flag is used by callers that are trying to make space for a
+ * new buffer in a full arc cache.
+ */
+static void *
+arc_evict(arc_state_t *state, int64_t bytes, boolean_t recycle,
+    arc_buf_contents_t type)
+{
+	arc_state_t *evicted_state;
+	uint64_t bytes_evicted = 0, skipped = 0, missed = 0;
+	arc_buf_hdr_t *ab, *ab_prev = NULL;
+	kmutex_t *hash_lock;
+	boolean_t have_lock;
+	void *stolen = NULL;
+
+	ASSERT(state == arc_mru || state == arc_mfu);
+
+	evicted_state = (state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;
+
+	mutex_enter(&state->arcs_mtx);
+	mutex_enter(&evicted_state->arcs_mtx);
+
+	for (ab = list_tail(&state->arcs_list); ab; ab = ab_prev) {
+		ab_prev = list_prev(&state->arcs_list, ab);
+		/* prefetch buffers have a minimum lifespan */
+		if (HDR_IO_IN_PROGRESS(ab) ||
+		    (ab->b_flags & (ARC_PREFETCH|ARC_INDIRECT) &&
+		    lbolt - ab->b_arc_access < arc_min_prefetch_lifespan)) {
+			skipped++;
+			continue;
+		}
+		/* "lookahead" for better eviction candidate */
+		if (recycle && ab->b_size != bytes &&
+		    ab_prev && ab_prev->b_size == bytes)
+			continue;
+		hash_lock = HDR_LOCK(ab);
+		have_lock = MUTEX_HELD(hash_lock);
+		if (have_lock || mutex_tryenter(hash_lock)) {
+			ASSERT3U(refcount_count(&ab->b_refcnt), ==, 0);
+			ASSERT(ab->b_datacnt > 0);
+			while (ab->b_buf) {
+				arc_buf_t *buf = ab->b_buf;
+				if (buf->b_data) {
+					bytes_evicted += ab->b_size;
+					if (recycle && ab->b_type == type &&
+					    ab->b_size == bytes) {
+						stolen = buf->b_data;
+						recycle = FALSE;
+					}
+				}
+				if (buf->b_efunc) {
+					mutex_enter(&arc_eviction_mtx);
+					arc_buf_destroy(buf,
+					    buf->b_data == stolen, FALSE);
+					ab->b_buf = buf->b_next;
+					buf->b_hdr = &arc_eviction_hdr;
+					buf->b_next = arc_eviction_list;
+					arc_eviction_list = buf;
+					mutex_exit(&arc_eviction_mtx);
+				} else {
+					arc_buf_destroy(buf,
+					    buf->b_data == stolen, TRUE);
+				}
+			}
+			ASSERT(ab->b_datacnt == 0);
+			arc_change_state(evicted_state, ab, hash_lock);
+			ASSERT(HDR_IN_HASH_TABLE(ab));
+			ab->b_flags = ARC_IN_HASH_TABLE;
+			DTRACE_PROBE1(arc__evict, arc_buf_hdr_t *, ab);
+			if (!have_lock)
+				mutex_exit(hash_lock);
+			if (bytes >= 0 && bytes_evicted >= bytes)
+				break;
+		} else {
+			missed += 1;
+		}
+	}
+
+	mutex_exit(&evicted_state->arcs_mtx);
+	mutex_exit(&state->arcs_mtx);
+
+	if (bytes_evicted < bytes)
+		dprintf("only evicted %lld bytes from %x",
+		    (longlong_t)bytes_evicted, state);
+
+	if (skipped)
+		ARCSTAT_INCR(arcstat_evict_skip, skipped);
+
+	if (missed)
+		ARCSTAT_INCR(arcstat_mutex_miss, missed);
+
+	return (stolen);
+}
+
+/*
+ * Remove buffers from list until we've removed the specified number of
+ * bytes.  Destroy the buffers that are removed.
+ */
+static void
+arc_evict_ghost(arc_state_t *state, int64_t bytes)
+{
+	arc_buf_hdr_t *ab, *ab_prev;
+	kmutex_t *hash_lock;
+	uint64_t bytes_deleted = 0;
+	uint64_t bufs_skipped = 0;
+
+	ASSERT(GHOST_STATE(state));
+top:
+	mutex_enter(&state->arcs_mtx);
+	for (ab = list_tail(&state->arcs_list); ab; ab = ab_prev) {
+		ab_prev = list_prev(&state->arcs_list, ab);
+		hash_lock = HDR_LOCK(ab);
+		if (mutex_tryenter(hash_lock)) {
+			ASSERT(!HDR_IO_IN_PROGRESS(ab));
+			ASSERT(ab->b_buf == NULL);
+			arc_change_state(arc_anon, ab, hash_lock);
+			mutex_exit(hash_lock);
+			ARCSTAT_BUMP(arcstat_deleted);
+			bytes_deleted += ab->b_size;
+			arc_hdr_destroy(ab);
+			DTRACE_PROBE1(arc__delete, arc_buf_hdr_t *, ab);
+			if (bytes >= 0 && bytes_deleted >= bytes)
+				break;
+		} else {
+			if (bytes < 0) {
+				mutex_exit(&state->arcs_mtx);
+				mutex_enter(hash_lock);
+				mutex_exit(hash_lock);
+				goto top;
+			}
+			bufs_skipped += 1;
+		}
+	}
+	mutex_exit(&state->arcs_mtx);
+
+	if (bufs_skipped) {
+		ARCSTAT_INCR(arcstat_mutex_miss, bufs_skipped);
+		ASSERT(bytes >= 0);
+	}
+
+	if (bytes_deleted < bytes)
+		dprintf("only deleted %lld bytes from %p",
+		    (longlong_t)bytes_deleted, state);
+}
+
+static void
+arc_adjust(void)
+{
+	int64_t top_sz, mru_over, arc_over, todelete;
+
+	top_sz = arc_anon->arcs_size + arc_mru->arcs_size;
+
+	if (top_sz > arc_p && arc_mru->arcs_lsize > 0) {
+		int64_t toevict = MIN(arc_mru->arcs_lsize, top_sz - arc_p);
+		(void) arc_evict(arc_mru, toevict, FALSE, ARC_BUFC_UNDEF);
+		top_sz = arc_anon->arcs_size + arc_mru->arcs_size;
+	}
+
+	mru_over = top_sz + arc_mru_ghost->arcs_size - arc_c;
+
+	if (mru_over > 0) {
+		if (arc_mru_ghost->arcs_lsize > 0) {
+			todelete = MIN(arc_mru_ghost->arcs_lsize, mru_over);
+			arc_evict_ghost(arc_mru_ghost, todelete);
+		}
+	}
+
+	if ((arc_over = arc_size - arc_c) > 0) {
+		int64_t tbl_over;
+
+		if (arc_mfu->arcs_lsize > 0) {
+			int64_t toevict = MIN(arc_mfu->arcs_lsize, arc_over);
+			(void) arc_evict(arc_mfu, toevict, FALSE,
+			    ARC_BUFC_UNDEF);
+		}
+
+		tbl_over = arc_size + arc_mru_ghost->arcs_lsize +
+		    arc_mfu_ghost->arcs_lsize - arc_c*2;
+
+		if (tbl_over > 0 && arc_mfu_ghost->arcs_lsize > 0) {
+			todelete = MIN(arc_mfu_ghost->arcs_lsize, tbl_over);
+			arc_evict_ghost(arc_mfu_ghost, todelete);
+		}
+	}
+}
+
+static void
+arc_do_user_evicts(void)
+{
+	mutex_enter(&arc_eviction_mtx);
+	while (arc_eviction_list != NULL) {
+		arc_buf_t *buf = arc_eviction_list;
+		arc_eviction_list = buf->b_next;
+		buf->b_hdr = NULL;
+		mutex_exit(&arc_eviction_mtx);
+
+		if (buf->b_efunc != NULL)
+			VERIFY(buf->b_efunc(buf) == 0);
+
+		buf->b_efunc = NULL;
+		buf->b_private = NULL;
+		kmem_cache_free(buf_cache, buf);
+		mutex_enter(&arc_eviction_mtx);
+	}
+	mutex_exit(&arc_eviction_mtx);
+}
+
+/*
+ * Flush all *evictable* data from the cache.
+ * NOTE: this will not touch "active" (i.e. referenced) data.
+ */
+void
+arc_flush(void)
+{
+	while (list_head(&arc_mru->arcs_list))
+		(void) arc_evict(arc_mru, -1, FALSE, ARC_BUFC_UNDEF);
+	while (list_head(&arc_mfu->arcs_list))
+		(void) arc_evict(arc_mfu, -1, FALSE, ARC_BUFC_UNDEF);
+
+	arc_evict_ghost(arc_mru_ghost, -1);
+	arc_evict_ghost(arc_mfu_ghost, -1);
+
+	mutex_enter(&arc_reclaim_thr_lock);
+	arc_do_user_evicts();
+	mutex_exit(&arc_reclaim_thr_lock);
+	ASSERT(arc_eviction_list == NULL);
+}
+
+int arc_shrink_shift = 5;		/* log2(fraction of arc to reclaim) */
+
+void
+arc_shrink(void)
+{
+	if (arc_c > arc_c_min) {
+		uint64_t to_free;
+
+#ifdef _KERNEL
+		to_free = arc_c >> arc_shrink_shift;
+#else
+		to_free = arc_c >> arc_shrink_shift;
+#endif
+		if (arc_c > arc_c_min + to_free)
+			atomic_add_64(&arc_c, -to_free);
+		else
+			arc_c = arc_c_min;
+
+		atomic_add_64(&arc_p, -(arc_p >> arc_shrink_shift));
+		if (arc_c > arc_size)
+			arc_c = MAX(arc_size, arc_c_min);
+		if (arc_p > arc_c)
+			arc_p = (arc_c >> 1);
+		ASSERT(arc_c >= arc_c_min);
+		ASSERT((int64_t)arc_p >= 0);
+	}
+
+	if (arc_size > arc_c)
+		arc_adjust();
+}
+
+static int zfs_needfree = 0;
+
+static int
+arc_reclaim_needed(void)
+{
+#if 0
+	uint64_t extra;
+#endif
+
+#ifdef _KERNEL
+
+	if (zfs_needfree)
+		return (1);
+
+#if 0
+	/*
+	 * check to make sure that swapfs has enough space so that anon
+	 * reservations can still succeeed. anon_resvmem() checks that the
+	 * availrmem is greater than swapfs_minfree, and the number of reserved
+	 * swap pages.  We also add a bit of extra here just to prevent
+	 * circumstances from getting really dire.
+	 */
+	if (availrmem < swapfs_minfree + swapfs_reserve + extra)
+		return (1);
+
+	/*
+	 * If zio data pages are being allocated out of a separate heap segment,
+	 * then check that the size of available vmem for this area remains
+	 * above 1/4th free.  This needs to be done when the size of the
+	 * non-default segment is smaller than physical memory, so we could
+	 * conceivably run out of VA in that segment before running out of
+	 * physical memory.
+	 */
+	if (zio_arena != NULL) {
+		size_t arc_ziosize =
+		    btop(vmem_size(zio_arena, VMEM_FREE | VMEM_ALLOC));
+
+		if ((physmem > arc_ziosize) &&
+		    (btop(vmem_size(zio_arena, VMEM_FREE)) < arc_ziosize >> 2))
+			return (1);
+	}
+
+#if defined(__i386)
+	/*
+	 * If we're on an i386 platform, it's possible that we'll exhaust the
+	 * kernel heap space before we ever run out of available physical
+	 * memory.  Most checks of the size of the heap_area compare against
+	 * tune.t_minarmem, which is the minimum available real memory that we
+	 * can have in the system.  However, this is generally fixed at 25 pages
+	 * which is so low that it's useless.  In this comparison, we seek to
+	 * calculate the total heap-size, and reclaim if more than 3/4ths of the
+	 * heap is allocated.  (Or, in the caclulation, if less than 1/4th is
+	 * free)
+	 */
+	if (btop(vmem_size(heap_arena, VMEM_FREE)) <
+	    (btop(vmem_size(heap_arena, VMEM_FREE | VMEM_ALLOC)) >> 2))
+		return (1);
+#endif
+#else
+	if (kmem_map->size > (vm_kmem_size * 3) / 4)
+		return (1);
+#endif
+
+#else
+	if (spa_get_random(100) == 0)
+		return (1);
+#endif
+	return (0);
+}
+
+static void
+arc_kmem_reap_now(arc_reclaim_strategy_t strat)
+{
+#ifdef ZIO_USE_UMA
+	size_t			i;
+	kmem_cache_t		*prev_cache = NULL;
+	kmem_cache_t		*prev_data_cache = NULL;
+	extern kmem_cache_t	*zio_buf_cache[];
+	extern kmem_cache_t	*zio_data_buf_cache[];
+#endif
+
+#ifdef _KERNEL
+	/*
+	 * First purge some DNLC entries, in case the DNLC is using
+	 * up too much memory.
+	 */
+	dnlc_reduce_cache((void *)(uintptr_t)arc_reduce_dnlc_percent);
+
+#if defined(__i386)
+	/*
+	 * Reclaim unused memory from all kmem caches.
+	 */
+	kmem_reap();
+#endif
+#endif
+
+	/*
+	 * An agressive reclamation will shrink the cache size as well as
+	 * reap free buffers from the arc kmem caches.
+	 */
+	if (strat == ARC_RECLAIM_AGGR)
+		arc_shrink();
+
+#ifdef ZIO_USE_UMA
+	for (i = 0; i < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; i++) {
+		if (zio_buf_cache[i] != prev_cache) {
+			prev_cache = zio_buf_cache[i];
+			kmem_cache_reap_now(zio_buf_cache[i]);
+		}
+		if (zio_data_buf_cache[i] != prev_data_cache) {
+			prev_data_cache = zio_data_buf_cache[i];
+			kmem_cache_reap_now(zio_data_buf_cache[i]);
+		}
+	}
+#endif
+	kmem_cache_reap_now(buf_cache);
+	kmem_cache_reap_now(hdr_cache);
+}
+
+static void
+arc_reclaim_thread(void *dummy __unused)
+{
+	clock_t			growtime = 0;
+	arc_reclaim_strategy_t	last_reclaim = ARC_RECLAIM_CONS;
+	callb_cpr_t		cpr;
+
+	CALLB_CPR_INIT(&cpr, &arc_reclaim_thr_lock, callb_generic_cpr, FTAG);
+
+	mutex_enter(&arc_reclaim_thr_lock);
+	while (arc_thread_exit == 0) {
+		if (arc_reclaim_needed()) {
+
+			if (arc_no_grow) {
+				if (last_reclaim == ARC_RECLAIM_CONS) {
+					last_reclaim = ARC_RECLAIM_AGGR;
+				} else {
+					last_reclaim = ARC_RECLAIM_CONS;
+				}
+			} else {
+				arc_no_grow = TRUE;
+				last_reclaim = ARC_RECLAIM_AGGR;
+				membar_producer();
+			}
+
+			/* reset the growth delay for every reclaim */
+			growtime = lbolt + (arc_grow_retry * hz);
+			ASSERT(growtime > 0);
+
+			if (zfs_needfree && last_reclaim == ARC_RECLAIM_CONS) {
+				/*
+				 * If zfs_needfree is TRUE our vm_lowmem hook
+				 * was called and in that case we must free some
+				 * memory, so switch to aggressive mode.
+				 */
+				arc_no_grow = TRUE;
+				last_reclaim = ARC_RECLAIM_AGGR;
+			}
+			arc_kmem_reap_now(last_reclaim);
+		} else if ((growtime > 0) && ((growtime - lbolt) <= 0)) {
+			arc_no_grow = FALSE;
+		}
+
+		if (zfs_needfree ||
+		    (2 * arc_c < arc_size +
+		    arc_mru_ghost->arcs_size + arc_mfu_ghost->arcs_size))
+			arc_adjust();
+
+		if (arc_eviction_list != NULL)
+			arc_do_user_evicts();
+
+		if (arc_reclaim_needed()) {
+			zfs_needfree = 0;
+#ifdef _KERNEL
+			wakeup(&zfs_needfree);
+#endif
+		}
+
+		/* block until needed, or one second, whichever is shorter */
+		CALLB_CPR_SAFE_BEGIN(&cpr);
+		(void) cv_timedwait(&arc_reclaim_thr_cv,
+		    &arc_reclaim_thr_lock, hz);
+		CALLB_CPR_SAFE_END(&cpr, &arc_reclaim_thr_lock);
+	}
+
+	arc_thread_exit = 0;
+	cv_broadcast(&arc_reclaim_thr_cv);
+	CALLB_CPR_EXIT(&cpr);		/* drops arc_reclaim_thr_lock */
+	thread_exit();
+}
+
+/*
+ * Adapt arc info given the number of bytes we are trying to add and
+ * the state that we are comming from.  This function is only called
+ * when we are adding new content to the cache.
+ */
+static void
+arc_adapt(int bytes, arc_state_t *state)
+{
+	int mult;
+
+	ASSERT(bytes > 0);
+	/*
+	 * Adapt the target size of the MRU list:
+	 *	- if we just hit in the MRU ghost list, then increase
+	 *	  the target size of the MRU list.
+	 *	- if we just hit in the MFU ghost list, then increase
+	 *	  the target size of the MFU list by decreasing the
+	 *	  target size of the MRU list.
+	 */
+	if (state == arc_mru_ghost) {
+		mult = ((arc_mru_ghost->arcs_size >= arc_mfu_ghost->arcs_size) ?
+		    1 : (arc_mfu_ghost->arcs_size/arc_mru_ghost->arcs_size));
+
+		arc_p = MIN(arc_c, arc_p + bytes * mult);
+	} else if (state == arc_mfu_ghost) {
+		mult = ((arc_mfu_ghost->arcs_size >= arc_mru_ghost->arcs_size) ?
+		    1 : (arc_mru_ghost->arcs_size/arc_mfu_ghost->arcs_size));
+
+		arc_p = MAX(0, (int64_t)arc_p - bytes * mult);
+	}
+	ASSERT((int64_t)arc_p >= 0);
+
+	if (arc_reclaim_needed()) {
+		cv_signal(&arc_reclaim_thr_cv);
+		return;
+	}
+
+	if (arc_no_grow)
+		return;
+
+	if (arc_c >= arc_c_max)
+		return;
+
+	/*
+	 * If we're within (2 * maxblocksize) bytes of the target
+	 * cache size, increment the target cache size
+	 */
+	if (arc_size > arc_c - (2ULL << SPA_MAXBLOCKSHIFT)) {
+		atomic_add_64(&arc_c, (int64_t)bytes);
+		if (arc_c > arc_c_max)
+			arc_c = arc_c_max;
+		else if (state == arc_anon)
+			atomic_add_64(&arc_p, (int64_t)bytes);
+		if (arc_p > arc_c)
+			arc_p = arc_c;
+	}
+	ASSERT((int64_t)arc_p >= 0);
+}
+
+/*
+ * Check if the cache has reached its limits and eviction is required
+ * prior to insert.
+ */
+static int
+arc_evict_needed()
+{
+	if (arc_reclaim_needed())
+		return (1);
+
+	return (arc_size > arc_c);
+}
+
+/*
+ * The buffer, supplied as the first argument, needs a data block.
+ * So, if we are at cache max, determine which cache should be victimized.
+ * We have the following cases:
+ *
+ * 1. Insert for MRU, p > sizeof(arc_anon + arc_mru) ->
+ * In this situation if we're out of space, but the resident size of the MFU is
+ * under the limit, victimize the MFU cache to satisfy this insertion request.
+ *
+ * 2. Insert for MRU, p <= sizeof(arc_anon + arc_mru) ->
+ * Here, we've used up all of the available space for the MRU, so we need to
+ * evict from our own cache instead.  Evict from the set of resident MRU
+ * entries.
+ *
+ * 3. Insert for MFU (c - p) > sizeof(arc_mfu) ->
+ * c minus p represents the MFU space in the cache, since p is the size of the
+ * cache that is dedicated to the MRU.  In this situation there's still space on
+ * the MFU side, so the MRU side needs to be victimized.
+ *
+ * 4. Insert for MFU (c - p) < sizeof(arc_mfu) ->
+ * MFU's resident set is consuming more space than it has been allotted.  In
+ * this situation, we must victimize our own cache, the MFU, for this insertion.
+ */
+static void
+arc_get_data_buf(arc_buf_t *buf)
+{
+	arc_state_t		*state = buf->b_hdr->b_state;
+	uint64_t		size = buf->b_hdr->b_size;
+	arc_buf_contents_t	type = buf->b_hdr->b_type;
+
+	arc_adapt(size, state);
+
+	/*
+	 * We have not yet reached cache maximum size,
+	 * just allocate a new buffer.
+	 */
+	if (!arc_evict_needed()) {
+		if (type == ARC_BUFC_METADATA) {
+			buf->b_data = zio_buf_alloc(size);
+		} else {
+			ASSERT(type == ARC_BUFC_DATA);
+			buf->b_data = zio_data_buf_alloc(size);
+		}
+		atomic_add_64(&arc_size, size);
+		goto out;
+	}
+
+	/*
+	 * If we are prefetching from the mfu ghost list, this buffer
+	 * will end up on the mru list; so steal space from there.
+	 */
+	if (state == arc_mfu_ghost)
+		state = buf->b_hdr->b_flags & ARC_PREFETCH ? arc_mru : arc_mfu;
+	else if (state == arc_mru_ghost)
+		state = arc_mru;
+
+	if (state == arc_mru || state == arc_anon) {
+		uint64_t mru_used = arc_anon->arcs_size + arc_mru->arcs_size;
+		state = (arc_p > mru_used) ? arc_mfu : arc_mru;
+	} else {
+		/* MFU cases */
+		uint64_t mfu_space = arc_c - arc_p;
+		state =  (mfu_space > arc_mfu->arcs_size) ? arc_mru : arc_mfu;
+	}
+	if ((buf->b_data = arc_evict(state, size, TRUE, type)) == NULL) {
+		if (type == ARC_BUFC_METADATA) {
+			buf->b_data = zio_buf_alloc(size);
+		} else {
+			ASSERT(type == ARC_BUFC_DATA);
+			buf->b_data = zio_data_buf_alloc(size);
+		}
+		atomic_add_64(&arc_size, size);
+		ARCSTAT_BUMP(arcstat_recycle_miss);
+	}
+	ASSERT(buf->b_data != NULL);
+out:
+	/*
+	 * Update the state size.  Note that ghost states have a
+	 * "ghost size" and so don't need to be updated.
+	 */
+	if (!GHOST_STATE(buf->b_hdr->b_state)) {
+		arc_buf_hdr_t *hdr = buf->b_hdr;
+
+		atomic_add_64(&hdr->b_state->arcs_size, size);
+		if (list_link_active(&hdr->b_arc_node)) {
+			ASSERT(refcount_is_zero(&hdr->b_refcnt));
+			atomic_add_64(&hdr->b_state->arcs_lsize, size);
+		}
+		/*
+		 * If we are growing the cache, and we are adding anonymous
+		 * data, and we have outgrown arc_p, update arc_p
+		 */
+		if (arc_size < arc_c && hdr->b_state == arc_anon &&
+		    arc_anon->arcs_size + arc_mru->arcs_size > arc_p)
+			arc_p = MIN(arc_c, arc_p + size);
+	}
+}
+
+/*
+ * This routine is called whenever a buffer is accessed.
+ * NOTE: the hash lock is dropped in this function.
+ */
+static void
+arc_access(arc_buf_hdr_t *buf, kmutex_t *hash_lock)
+{
+	ASSERT(MUTEX_HELD(hash_lock));
+
+	if (buf->b_state == arc_anon) {
+		/*
+		 * This buffer is not in the cache, and does not
+		 * appear in our "ghost" list.  Add the new buffer
+		 * to the MRU state.
+		 */
+
+		ASSERT(buf->b_arc_access == 0);
+		buf->b_arc_access = lbolt;
+		DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, buf);
+		arc_change_state(arc_mru, buf, hash_lock);
+
+	} else if (buf->b_state == arc_mru) {
+		/*
+		 * If this buffer is here because of a prefetch, then either:
+		 * - clear the flag if this is a "referencing" read
+		 *   (any subsequent access will bump this into the MFU state).
+		 * or
+		 * - move the buffer to the head of the list if this is
+		 *   another prefetch (to make it less likely to be evicted).
+		 */
+		if ((buf->b_flags & ARC_PREFETCH) != 0) {
+			if (refcount_count(&buf->b_refcnt) == 0) {
+				ASSERT(list_link_active(&buf->b_arc_node));
+				mutex_enter(&arc_mru->arcs_mtx);
+				list_remove(&arc_mru->arcs_list, buf);
+				list_insert_head(&arc_mru->arcs_list, buf);
+				mutex_exit(&arc_mru->arcs_mtx);
+			} else {
+				buf->b_flags &= ~ARC_PREFETCH;
+				ARCSTAT_BUMP(arcstat_mru_hits);
+			}
+			buf->b_arc_access = lbolt;
+			return;
+		}
+
+		/*
+		 * This buffer has been "accessed" only once so far,
+		 * but it is still in the cache. Move it to the MFU
+		 * state.
+		 */
+		if (lbolt > buf->b_arc_access + ARC_MINTIME) {
+			/*
+			 * More than 125ms have passed since we
+			 * instantiated this buffer.  Move it to the
+			 * most frequently used state.
+			 */
+			buf->b_arc_access = lbolt;
+			DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
+			arc_change_state(arc_mfu, buf, hash_lock);
+		}
+		ARCSTAT_BUMP(arcstat_mru_hits);
+	} else if (buf->b_state == arc_mru_ghost) {
+		arc_state_t	*new_state;
+		/*
+		 * This buffer has been "accessed" recently, but
+		 * was evicted from the cache.  Move it to the
+		 * MFU state.
+		 */
+
+		if (buf->b_flags & ARC_PREFETCH) {
+			new_state = arc_mru;
+			if (refcount_count(&buf->b_refcnt) > 0)
+				buf->b_flags &= ~ARC_PREFETCH;
+			DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, buf);
+		} else {
+			new_state = arc_mfu;
+			DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
+		}
+
+		buf->b_arc_access = lbolt;
+		arc_change_state(new_state, buf, hash_lock);
+
+		ARCSTAT_BUMP(arcstat_mru_ghost_hits);
+	} else if (buf->b_state == arc_mfu) {
+		/*
+		 * This buffer has been accessed more than once and is
+		 * still in the cache.  Keep it in the MFU state.
+		 *
+		 * NOTE: an add_reference() that occurred when we did
+		 * the arc_read() will have kicked this off the list.
+		 * If it was a prefetch, we will explicitly move it to
+		 * the head of the list now.
+		 */
+		if ((buf->b_flags & ARC_PREFETCH) != 0) {
+			ASSERT(refcount_count(&buf->b_refcnt) == 0);
+			ASSERT(list_link_active(&buf->b_arc_node));
+			mutex_enter(&arc_mfu->arcs_mtx);
+			list_remove(&arc_mfu->arcs_list, buf);
+			list_insert_head(&arc_mfu->arcs_list, buf);
+			mutex_exit(&arc_mfu->arcs_mtx);
+		}
+		ARCSTAT_BUMP(arcstat_mfu_hits);
+		buf->b_arc_access = lbolt;
+	} else if (buf->b_state == arc_mfu_ghost) {
+		arc_state_t	*new_state = arc_mfu;
+		/*
+		 * This buffer has been accessed more than once but has
+		 * been evicted from the cache.  Move it back to the
+		 * MFU state.
+		 */
+
+		if (buf->b_flags & ARC_PREFETCH) {
+			/*
+			 * This is a prefetch access...
+			 * move this block back to the MRU state.
+			 */
+			ASSERT3U(refcount_count(&buf->b_refcnt), ==, 0);
+			new_state = arc_mru;
+		}
+
+		buf->b_arc_access = lbolt;
+		DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
+		arc_change_state(new_state, buf, hash_lock);
+
+		ARCSTAT_BUMP(arcstat_mfu_ghost_hits);
+	} else {
+		ASSERT(!"invalid arc state");
+	}
+}
+
+/* a generic arc_done_func_t which you can use */
+/* ARGSUSED */
+void
+arc_bcopy_func(zio_t *zio, arc_buf_t *buf, void *arg)
+{
+	bcopy(buf->b_data, arg, buf->b_hdr->b_size);
+	VERIFY(arc_buf_remove_ref(buf, arg) == 1);
+}
+
+/* a generic arc_done_func_t which you can use */
+void
+arc_getbuf_func(zio_t *zio, arc_buf_t *buf, void *arg)
+{
+	arc_buf_t **bufp = arg;
+	if (zio && zio->io_error) {
+		VERIFY(arc_buf_remove_ref(buf, arg) == 1);
+		*bufp = NULL;
+	} else {
+		*bufp = buf;
+	}
+}
+
+static void
+arc_read_done(zio_t *zio)
+{
+	arc_buf_hdr_t	*hdr, *found;
+	arc_buf_t	*buf;
+	arc_buf_t	*abuf;	/* buffer we're assigning to callback */
+	kmutex_t	*hash_lock;
+	arc_callback_t	*callback_list, *acb;
+	int		freeable = FALSE;
+
+	buf = zio->io_private;
+	hdr = buf->b_hdr;
+
+	/*
+	 * The hdr was inserted into hash-table and removed from lists
+	 * prior to starting I/O.  We should find this header, since
+	 * it's in the hash table, and it should be legit since it's
+	 * not possible to evict it during the I/O.  The only possible
+	 * reason for it not to be found is if we were freed during the
+	 * read.
+	 */
+	found = buf_hash_find(zio->io_spa, &hdr->b_dva, hdr->b_birth,
+	    &hash_lock);
+
+	ASSERT((found == NULL && HDR_FREED_IN_READ(hdr) && hash_lock == NULL) ||
+	    (found == hdr && DVA_EQUAL(&hdr->b_dva, BP_IDENTITY(zio->io_bp))));
+
+	/* byteswap if necessary */
+	callback_list = hdr->b_acb;
+	ASSERT(callback_list != NULL);
+	if (BP_SHOULD_BYTESWAP(zio->io_bp) && callback_list->acb_byteswap)
+		callback_list->acb_byteswap(buf->b_data, hdr->b_size);
+
+	arc_cksum_compute(buf);
+
+	/* create copies of the data buffer for the callers */
+	abuf = buf;
+	for (acb = callback_list; acb; acb = acb->acb_next) {
+		if (acb->acb_done) {
+			if (abuf == NULL)
+				abuf = arc_buf_clone(buf);
+			acb->acb_buf = abuf;
+			abuf = NULL;
+		}
+	}
+	hdr->b_acb = NULL;
+	hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
+	ASSERT(!HDR_BUF_AVAILABLE(hdr));
+	if (abuf == buf)
+		hdr->b_flags |= ARC_BUF_AVAILABLE;
+
+	ASSERT(refcount_is_zero(&hdr->b_refcnt) || callback_list != NULL);
+
+	if (zio->io_error != 0) {
+		hdr->b_flags |= ARC_IO_ERROR;
+		if (hdr->b_state != arc_anon)
+			arc_change_state(arc_anon, hdr, hash_lock);
+		if (HDR_IN_HASH_TABLE(hdr))
+			buf_hash_remove(hdr);
+		freeable = refcount_is_zero(&hdr->b_refcnt);
+		/* convert checksum errors into IO errors */
+		if (zio->io_error == ECKSUM)
+			zio->io_error = EIO;
+	}
+
+	/*
+	 * Broadcast before we drop the hash_lock to avoid the possibility
+	 * that the hdr (and hence the cv) might be freed before we get to
+	 * the cv_broadcast().
+	 */
+	cv_broadcast(&hdr->b_cv);
+
+	if (hash_lock) {
+		/*
+		 * Only call arc_access on anonymous buffers.  This is because
+		 * if we've issued an I/O for an evicted buffer, we've already
+		 * called arc_access (to prevent any simultaneous readers from
+		 * getting confused).
+		 */
+		if (zio->io_error == 0 && hdr->b_state == arc_anon)
+			arc_access(hdr, hash_lock);
+		mutex_exit(hash_lock);
+	} else {
+		/*
+		 * This block was freed while we waited for the read to
+		 * complete.  It has been removed from the hash table and
+		 * moved to the anonymous state (so that it won't show up
+		 * in the cache).
+		 */
+		ASSERT3P(hdr->b_state, ==, arc_anon);
+		freeable = refcount_is_zero(&hdr->b_refcnt);
+	}
+
+	/* execute each callback and free its structure */
+	while ((acb = callback_list) != NULL) {
+		if (acb->acb_done)
+			acb->acb_done(zio, acb->acb_buf, acb->acb_private);
+
+		if (acb->acb_zio_dummy != NULL) {
+			acb->acb_zio_dummy->io_error = zio->io_error;
+			zio_nowait(acb->acb_zio_dummy);
+		}
+
+		callback_list = acb->acb_next;
+		kmem_free(acb, sizeof (arc_callback_t));
+	}
+
+	if (freeable)
+		arc_hdr_destroy(hdr);
+}
+
+/*
+ * "Read" the block block at the specified DVA (in bp) via the
+ * cache.  If the block is found in the cache, invoke the provided
+ * callback immediately and return.  Note that the `zio' parameter
+ * in the callback will be NULL in this case, since no IO was
+ * required.  If the block is not in the cache pass the read request
+ * on to the spa with a substitute callback function, so that the
+ * requested block will be added to the cache.
+ *
+ * If a read request arrives for a block that has a read in-progress,
+ * either wait for the in-progress read to complete (and return the
+ * results); or, if this is a read with a "done" func, add a record
+ * to the read to invoke the "done" func when the read completes,
+ * and return; or just return.
+ *
+ * arc_read_done() will invoke all the requested "done" functions
+ * for readers of this block.
+ */
+int
+arc_read(zio_t *pio, spa_t *spa, blkptr_t *bp, arc_byteswap_func_t *swap,
+    arc_done_func_t *done, void *private, int priority, int flags,
+    uint32_t *arc_flags, zbookmark_t *zb)
+{
+	arc_buf_hdr_t *hdr;
+	arc_buf_t *buf;
+	kmutex_t *hash_lock;
+	zio_t	*rzio;
+
+top:
+	hdr = buf_hash_find(spa, BP_IDENTITY(bp), bp->blk_birth, &hash_lock);
+	if (hdr && hdr->b_datacnt > 0) {
+
+		*arc_flags |= ARC_CACHED;
+
+		if (HDR_IO_IN_PROGRESS(hdr)) {
+
+			if (*arc_flags & ARC_WAIT) {
+				cv_wait(&hdr->b_cv, hash_lock);
+				mutex_exit(hash_lock);
+				goto top;
+			}
+			ASSERT(*arc_flags & ARC_NOWAIT);
+
+			if (done) {
+				arc_callback_t	*acb = NULL;
+
+				acb = kmem_zalloc(sizeof (arc_callback_t),
+				    KM_SLEEP);
+				acb->acb_done = done;
+				acb->acb_private = private;
+				acb->acb_byteswap = swap;
+				if (pio != NULL)
+					acb->acb_zio_dummy = zio_null(pio,
+					    spa, NULL, NULL, flags);
+
+				ASSERT(acb->acb_done != NULL);
+				acb->acb_next = hdr->b_acb;
+				hdr->b_acb = acb;
+				add_reference(hdr, hash_lock, private);
+				mutex_exit(hash_lock);
+				return (0);
+			}
+			mutex_exit(hash_lock);
+			return (0);
+		}
+
+		ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);
+
+		if (done) {
+			add_reference(hdr, hash_lock, private);
+			/*
+			 * If this block is already in use, create a new
+			 * copy of the data so that we will be guaranteed
+			 * that arc_release() will always succeed.
+			 */
+			buf = hdr->b_buf;
+			ASSERT(buf);
+			ASSERT(buf->b_data);
+			if (HDR_BUF_AVAILABLE(hdr)) {
+				ASSERT(buf->b_efunc == NULL);
+				hdr->b_flags &= ~ARC_BUF_AVAILABLE;
+			} else {
+				buf = arc_buf_clone(buf);
+			}
+		} else if (*arc_flags & ARC_PREFETCH &&
+		    refcount_count(&hdr->b_refcnt) == 0) {
+			hdr->b_flags |= ARC_PREFETCH;
+		}
+		DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
+		arc_access(hdr, hash_lock);
+		mutex_exit(hash_lock);
+		ARCSTAT_BUMP(arcstat_hits);
+		ARCSTAT_CONDSTAT(!(hdr->b_flags & ARC_PREFETCH),
+		    demand, prefetch, hdr->b_type != ARC_BUFC_METADATA,
+		    data, metadata, hits);
+
+		if (done)
+			done(NULL, buf, private);
+	} else {
+		uint64_t size = BP_GET_LSIZE(bp);
+		arc_callback_t	*acb;
+
+		if (hdr == NULL) {
+			/* this block is not in the cache */
+			arc_buf_hdr_t	*exists;
+			arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
+			buf = arc_buf_alloc(spa, size, private, type);
+			hdr = buf->b_hdr;
+			hdr->b_dva = *BP_IDENTITY(bp);
+			hdr->b_birth = bp->blk_birth;
+			hdr->b_cksum0 = bp->blk_cksum.zc_word[0];
+			exists = buf_hash_insert(hdr, &hash_lock);
+			if (exists) {
+				/* somebody beat us to the hash insert */
+				mutex_exit(hash_lock);
+				bzero(&hdr->b_dva, sizeof (dva_t));
+				hdr->b_birth = 0;
+				hdr->b_cksum0 = 0;
+				(void) arc_buf_remove_ref(buf, private);
+				goto top; /* restart the IO request */
+			}
+			/* if this is a prefetch, we don't have a reference */
+			if (*arc_flags & ARC_PREFETCH) {
+				(void) remove_reference(hdr, hash_lock,
+				    private);
+				hdr->b_flags |= ARC_PREFETCH;
+			}
+			if (BP_GET_LEVEL(bp) > 0)
+				hdr->b_flags |= ARC_INDIRECT;
+		} else {
+			/* this block is in the ghost cache */
+			ASSERT(GHOST_STATE(hdr->b_state));
+			ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+			ASSERT3U(refcount_count(&hdr->b_refcnt), ==, 0);
+			ASSERT(hdr->b_buf == NULL);
+
+			/* if this is a prefetch, we don't have a reference */
+			if (*arc_flags & ARC_PREFETCH)
+				hdr->b_flags |= ARC_PREFETCH;
+			else
+				add_reference(hdr, hash_lock, private);
+			buf = kmem_cache_alloc(buf_cache, KM_SLEEP);
+			buf->b_hdr = hdr;
+			buf->b_data = NULL;
+			buf->b_efunc = NULL;
+			buf->b_private = NULL;
+			buf->b_next = NULL;
+			hdr->b_buf = buf;
+			arc_get_data_buf(buf);
+			ASSERT(hdr->b_datacnt == 0);
+			hdr->b_datacnt = 1;
+
+		}
+
+		acb = kmem_zalloc(sizeof (arc_callback_t), KM_SLEEP);
+		acb->acb_done = done;
+		acb->acb_private = private;
+		acb->acb_byteswap = swap;
+
+		ASSERT(hdr->b_acb == NULL);
+		hdr->b_acb = acb;
+		hdr->b_flags |= ARC_IO_IN_PROGRESS;
+
+		/*
+		 * If the buffer has been evicted, migrate it to a present state
+		 * before issuing the I/O.  Once we drop the hash-table lock,
+		 * the header will be marked as I/O in progress and have an
+		 * attached buffer.  At this point, anybody who finds this
+		 * buffer ought to notice that it's legit but has a pending I/O.
+		 */
+
+		if (GHOST_STATE(hdr->b_state))
+			arc_access(hdr, hash_lock);
+		mutex_exit(hash_lock);
+
+		ASSERT3U(hdr->b_size, ==, size);
+		DTRACE_PROBE3(arc__miss, blkptr_t *, bp, uint64_t, size,
+		    zbookmark_t *, zb);
+		ARCSTAT_BUMP(arcstat_misses);
+		ARCSTAT_CONDSTAT(!(hdr->b_flags & ARC_PREFETCH),
+		    demand, prefetch, hdr->b_type != ARC_BUFC_METADATA,
+		    data, metadata, misses);
+
+		rzio = zio_read(pio, spa, bp, buf->b_data, size,
+		    arc_read_done, buf, priority, flags, zb);
+
+		if (*arc_flags & ARC_WAIT)
+			return (zio_wait(rzio));
+
+		ASSERT(*arc_flags & ARC_NOWAIT);
+		zio_nowait(rzio);
+	}
+	return (0);
+}
+
+/*
+ * arc_read() variant to support pool traversal.  If the block is already
+ * in the ARC, make a copy of it; otherwise, the caller will do the I/O.
+ * The idea is that we don't want pool traversal filling up memory, but
+ * if the ARC already has the data anyway, we shouldn't pay for the I/O.
+ */
+int
+arc_tryread(spa_t *spa, blkptr_t *bp, void *data)
+{
+	arc_buf_hdr_t *hdr;
+	kmutex_t *hash_mtx;
+	int rc = 0;
+
+	hdr = buf_hash_find(spa, BP_IDENTITY(bp), bp->blk_birth, &hash_mtx);
+
+	if (hdr && hdr->b_datacnt > 0 && !HDR_IO_IN_PROGRESS(hdr)) {
+		arc_buf_t *buf = hdr->b_buf;
+
+		ASSERT(buf);
+		while (buf->b_data == NULL) {
+			buf = buf->b_next;
+			ASSERT(buf);
+		}
+		bcopy(buf->b_data, data, hdr->b_size);
+	} else {
+		rc = ENOENT;
+	}
+
+	if (hash_mtx)
+		mutex_exit(hash_mtx);
+
+	return (rc);
+}
+
+void
+arc_set_callback(arc_buf_t *buf, arc_evict_func_t *func, void *private)
+{
+	ASSERT(buf->b_hdr != NULL);
+	ASSERT(buf->b_hdr->b_state != arc_anon);
+	ASSERT(!refcount_is_zero(&buf->b_hdr->b_refcnt) || func == NULL);
+	buf->b_efunc = func;
+	buf->b_private = private;
+}
+
+/*
+ * This is used by the DMU to let the ARC know that a buffer is
+ * being evicted, so the ARC should clean up.  If this arc buf
+ * is not yet in the evicted state, it will be put there.
+ */
+int
+arc_buf_evict(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr;
+	kmutex_t *hash_lock;
+	arc_buf_t **bufp;
+
+	mutex_enter(&arc_eviction_mtx);
+	hdr = buf->b_hdr;
+	if (hdr == NULL) {
+		/*
+		 * We are in arc_do_user_evicts().
+		 */
+		ASSERT(buf->b_data == NULL);
+		mutex_exit(&arc_eviction_mtx);
+		return (0);
+	}
+	hash_lock = HDR_LOCK(hdr);
+	mutex_exit(&arc_eviction_mtx);
+
+	mutex_enter(hash_lock);
+
+	if (buf->b_data == NULL) {
+		/*
+		 * We are on the eviction list.
+		 */
+		mutex_exit(hash_lock);
+		mutex_enter(&arc_eviction_mtx);
+		if (buf->b_hdr == NULL) {
+			/*
+			 * We are already in arc_do_user_evicts().
+			 */
+			mutex_exit(&arc_eviction_mtx);
+			return (0);
+		} else {
+			arc_buf_t copy = *buf; /* structure assignment */
+			/*
+			 * Process this buffer now
+			 * but let arc_do_user_evicts() do the reaping.
+			 */
+			buf->b_efunc = NULL;
+			mutex_exit(&arc_eviction_mtx);
+			VERIFY(copy.b_efunc(&copy) == 0);
+			return (1);
+		}
+	}
+
+	ASSERT(buf->b_hdr == hdr);
+	ASSERT3U(refcount_count(&hdr->b_refcnt), <, hdr->b_datacnt);
+	ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);
+
+	/*
+	 * Pull this buffer off of the hdr
+	 */
+	bufp = &hdr->b_buf;
+	while (*bufp != buf)
+		bufp = &(*bufp)->b_next;
+	*bufp = buf->b_next;
+
+	ASSERT(buf->b_data != NULL);
+	arc_buf_destroy(buf, FALSE, FALSE);
+
+	if (hdr->b_datacnt == 0) {
+		arc_state_t *old_state = hdr->b_state;
+		arc_state_t *evicted_state;
+
+		ASSERT(refcount_is_zero(&hdr->b_refcnt));
+
+		evicted_state =
+		    (old_state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;
+
+		mutex_enter(&old_state->arcs_mtx);
+		mutex_enter(&evicted_state->arcs_mtx);
+
+		arc_change_state(evicted_state, hdr, hash_lock);
+		ASSERT(HDR_IN_HASH_TABLE(hdr));
+		hdr->b_flags = ARC_IN_HASH_TABLE;
+
+		mutex_exit(&evicted_state->arcs_mtx);
+		mutex_exit(&old_state->arcs_mtx);
+	}
+	mutex_exit(hash_lock);
+
+	VERIFY(buf->b_efunc(buf) == 0);
+	buf->b_efunc = NULL;
+	buf->b_private = NULL;
+	buf->b_hdr = NULL;
+	kmem_cache_free(buf_cache, buf);
+	return (1);
+}
+
+/*
+ * Release this buffer from the cache.  This must be done
+ * after a read and prior to modifying the buffer contents.
+ * If the buffer has more than one reference, we must make
+ * make a new hdr for the buffer.
+ */
+void
+arc_release(arc_buf_t *buf, void *tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	kmutex_t *hash_lock = HDR_LOCK(hdr);
+
+	/* this buffer is not on any list */
+	ASSERT(refcount_count(&hdr->b_refcnt) > 0);
+
+	if (hdr->b_state == arc_anon) {
+		/* this buffer is already released */
+		ASSERT3U(refcount_count(&hdr->b_refcnt), ==, 1);
+		ASSERT(BUF_EMPTY(hdr));
+		ASSERT(buf->b_efunc == NULL);
+		arc_buf_thaw(buf);
+		return;
+	}
+
+	mutex_enter(hash_lock);
+
+	/*
+	 * Do we have more than one buf?
+	 */
+	if (hdr->b_buf != buf || buf->b_next != NULL) {
+		arc_buf_hdr_t *nhdr;
+		arc_buf_t **bufp;
+		uint64_t blksz = hdr->b_size;
+		spa_t *spa = hdr->b_spa;
+		arc_buf_contents_t type = hdr->b_type;
+
+		ASSERT(hdr->b_datacnt > 1);
+		/*
+		 * Pull the data off of this buf and attach it to
+		 * a new anonymous buf.
+		 */
+		(void) remove_reference(hdr, hash_lock, tag);
+		bufp = &hdr->b_buf;
+		while (*bufp != buf)
+			bufp = &(*bufp)->b_next;
+		*bufp = (*bufp)->b_next;
+		buf->b_next = NULL;
+
+		ASSERT3U(hdr->b_state->arcs_size, >=, hdr->b_size);
+		atomic_add_64(&hdr->b_state->arcs_size, -hdr->b_size);
+		if (refcount_is_zero(&hdr->b_refcnt)) {
+			ASSERT3U(hdr->b_state->arcs_lsize, >=, hdr->b_size);
+			atomic_add_64(&hdr->b_state->arcs_lsize, -hdr->b_size);
+		}
+		hdr->b_datacnt -= 1;
+		arc_cksum_verify(buf);
+
+		mutex_exit(hash_lock);
+
+		nhdr = kmem_cache_alloc(hdr_cache, KM_SLEEP);
+		nhdr->b_size = blksz;
+		nhdr->b_spa = spa;
+		nhdr->b_type = type;
+		nhdr->b_buf = buf;
+		nhdr->b_state = arc_anon;
+		nhdr->b_arc_access = 0;
+		nhdr->b_flags = 0;
+		nhdr->b_datacnt = 1;
+		nhdr->b_freeze_cksum = NULL;
+		(void) refcount_add(&nhdr->b_refcnt, tag);
+		buf->b_hdr = nhdr;
+		atomic_add_64(&arc_anon->arcs_size, blksz);
+
+		hdr = nhdr;
+	} else {
+		ASSERT(refcount_count(&hdr->b_refcnt) == 1);
+		ASSERT(!list_link_active(&hdr->b_arc_node));
+		ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+		arc_change_state(arc_anon, hdr, hash_lock);
+		hdr->b_arc_access = 0;
+		mutex_exit(hash_lock);
+		bzero(&hdr->b_dva, sizeof (dva_t));
+		hdr->b_birth = 0;
+		hdr->b_cksum0 = 0;
+		arc_buf_thaw(buf);
+	}
+	buf->b_efunc = NULL;
+	buf->b_private = NULL;
+}
+
+int
+arc_released(arc_buf_t *buf)
+{
+	return (buf->b_data != NULL && buf->b_hdr->b_state == arc_anon);
+}
+
+int
+arc_has_callback(arc_buf_t *buf)
+{
+	return (buf->b_efunc != NULL);
+}
+
+#ifdef ZFS_DEBUG
+int
+arc_referenced(arc_buf_t *buf)
+{
+	return (refcount_count(&buf->b_hdr->b_refcnt));
+}
+#endif
+
+static void
+arc_write_ready(zio_t *zio)
+{
+	arc_write_callback_t *callback = zio->io_private;
+	arc_buf_t *buf = callback->awcb_buf;
+
+	if (callback->awcb_ready) {
+		ASSERT(!refcount_is_zero(&buf->b_hdr->b_refcnt));
+		callback->awcb_ready(zio, buf, callback->awcb_private);
+	}
+	arc_cksum_compute(buf);
+}
+
+static void
+arc_write_done(zio_t *zio)
+{
+	arc_write_callback_t *callback = zio->io_private;
+	arc_buf_t *buf = callback->awcb_buf;
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	hdr->b_acb = NULL;
+
+	/* this buffer is on no lists and is not in the hash table */
+	ASSERT3P(hdr->b_state, ==, arc_anon);
+
+	hdr->b_dva = *BP_IDENTITY(zio->io_bp);
+	hdr->b_birth = zio->io_bp->blk_birth;
+	hdr->b_cksum0 = zio->io_bp->blk_cksum.zc_word[0];
+	/*
+	 * If the block to be written was all-zero, we may have
+	 * compressed it away.  In this case no write was performed
+	 * so there will be no dva/birth-date/checksum.  The buffer
+	 * must therefor remain anonymous (and uncached).
+	 */
+	if (!BUF_EMPTY(hdr)) {
+		arc_buf_hdr_t *exists;
+		kmutex_t *hash_lock;
+
+		arc_cksum_verify(buf);
+
+		exists = buf_hash_insert(hdr, &hash_lock);
+		if (exists) {
+			/*
+			 * This can only happen if we overwrite for
+			 * sync-to-convergence, because we remove
+			 * buffers from the hash table when we arc_free().
+			 */
+			ASSERT(DVA_EQUAL(BP_IDENTITY(&zio->io_bp_orig),
+			    BP_IDENTITY(zio->io_bp)));
+			ASSERT3U(zio->io_bp_orig.blk_birth, ==,
+			    zio->io_bp->blk_birth);
+
+			ASSERT(refcount_is_zero(&exists->b_refcnt));
+			arc_change_state(arc_anon, exists, hash_lock);
+			mutex_exit(hash_lock);
+			arc_hdr_destroy(exists);
+			exists = buf_hash_insert(hdr, &hash_lock);
+			ASSERT3P(exists, ==, NULL);
+		}
+		hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
+		arc_access(hdr, hash_lock);
+		mutex_exit(hash_lock);
+	} else if (callback->awcb_done == NULL) {
+		int destroy_hdr;
+		/*
+		 * This is an anonymous buffer with no user callback,
+		 * destroy it if there are no active references.
+		 */
+		mutex_enter(&arc_eviction_mtx);
+		destroy_hdr = refcount_is_zero(&hdr->b_refcnt);
+		hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
+		mutex_exit(&arc_eviction_mtx);
+		if (destroy_hdr)
+			arc_hdr_destroy(hdr);
+	} else {
+		hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
+	}
+
+	if (callback->awcb_done) {
+		ASSERT(!refcount_is_zero(&hdr->b_refcnt));
+		callback->awcb_done(zio, buf, callback->awcb_private);
+	}
+
+	kmem_free(callback, sizeof (arc_write_callback_t));
+}
+
+zio_t *
+arc_write(zio_t *pio, spa_t *spa, int checksum, int compress, int ncopies,
+    uint64_t txg, blkptr_t *bp, arc_buf_t *buf,
+    arc_done_func_t *ready, arc_done_func_t *done, void *private, int priority,
+    int flags, zbookmark_t *zb)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	arc_write_callback_t *callback;
+	zio_t	*zio;
+
+	/* this is a private buffer - no locking required */
+	ASSERT3P(hdr->b_state, ==, arc_anon);
+	ASSERT(BUF_EMPTY(hdr));
+	ASSERT(!HDR_IO_ERROR(hdr));
+	ASSERT((hdr->b_flags & ARC_IO_IN_PROGRESS) == 0);
+	ASSERT(hdr->b_acb == 0);
+	callback = kmem_zalloc(sizeof (arc_write_callback_t), KM_SLEEP);
+	callback->awcb_ready = ready;
+	callback->awcb_done = done;
+	callback->awcb_private = private;
+	callback->awcb_buf = buf;
+	hdr->b_flags |= ARC_IO_IN_PROGRESS;
+	zio = zio_write(pio, spa, checksum, compress, ncopies, txg, bp,
+	    buf->b_data, hdr->b_size, arc_write_ready, arc_write_done, callback,
+	    priority, flags, zb);
+
+	return (zio);
+}
+
+int
+arc_free(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
+    zio_done_func_t *done, void *private, uint32_t arc_flags)
+{
+	arc_buf_hdr_t *ab;
+	kmutex_t *hash_lock;
+	zio_t	*zio;
+
+	/*
+	 * If this buffer is in the cache, release it, so it
+	 * can be re-used.
+	 */
+	ab = buf_hash_find(spa, BP_IDENTITY(bp), bp->blk_birth, &hash_lock);
+	if (ab != NULL) {
+		/*
+		 * The checksum of blocks to free is not always
+		 * preserved (eg. on the deadlist).  However, if it is
+		 * nonzero, it should match what we have in the cache.
+		 */
+		ASSERT(bp->blk_cksum.zc_word[0] == 0 ||
+		    ab->b_cksum0 == bp->blk_cksum.zc_word[0]);
+		if (ab->b_state != arc_anon)
+			arc_change_state(arc_anon, ab, hash_lock);
+		if (HDR_IO_IN_PROGRESS(ab)) {
+			/*
+			 * This should only happen when we prefetch.
+			 */
+			ASSERT(ab->b_flags & ARC_PREFETCH);
+			ASSERT3U(ab->b_datacnt, ==, 1);
+			ab->b_flags |= ARC_FREED_IN_READ;
+			if (HDR_IN_HASH_TABLE(ab))
+				buf_hash_remove(ab);
+			ab->b_arc_access = 0;
+			bzero(&ab->b_dva, sizeof (dva_t));
+			ab->b_birth = 0;
+			ab->b_cksum0 = 0;
+			ab->b_buf->b_efunc = NULL;
+			ab->b_buf->b_private = NULL;
+			mutex_exit(hash_lock);
+		} else if (refcount_is_zero(&ab->b_refcnt)) {
+			mutex_exit(hash_lock);
+			arc_hdr_destroy(ab);
+			ARCSTAT_BUMP(arcstat_deleted);
+		} else {
+			/*
+			 * We still have an active reference on this
+			 * buffer.  This can happen, e.g., from
+			 * dbuf_unoverride().
+			 */
+			ASSERT(!HDR_IN_HASH_TABLE(ab));
+			ab->b_arc_access = 0;
+			bzero(&ab->b_dva, sizeof (dva_t));
+			ab->b_birth = 0;
+			ab->b_cksum0 = 0;
+			ab->b_buf->b_efunc = NULL;
+			ab->b_buf->b_private = NULL;
+			mutex_exit(hash_lock);
+		}
+	}
+
+	zio = zio_free(pio, spa, txg, bp, done, private);
+
+	if (arc_flags & ARC_WAIT)
+		return (zio_wait(zio));
+
+	ASSERT(arc_flags & ARC_NOWAIT);
+	zio_nowait(zio);
+
+	return (0);
+}
+
+void
+arc_tempreserve_clear(uint64_t tempreserve)
+{
+	atomic_add_64(&arc_tempreserve, -tempreserve);
+	ASSERT((int64_t)arc_tempreserve >= 0);
+}
+
+int
+arc_tempreserve_space(uint64_t tempreserve)
+{
+#ifdef ZFS_DEBUG
+	/*
+	 * Once in a while, fail for no reason.  Everything should cope.
+	 */
+	if (spa_get_random(10000) == 0) {
+		dprintf("forcing random failure\n");
+		return (ERESTART);
+	}
+#endif
+	if (tempreserve > arc_c/4 && !arc_no_grow)
+		arc_c = MIN(arc_c_max, tempreserve * 4);
+	if (tempreserve > arc_c)
+		return (ENOMEM);
+
+	/*
+	 * Throttle writes when the amount of dirty data in the cache
+	 * gets too large.  We try to keep the cache less than half full
+	 * of dirty blocks so that our sync times don't grow too large.
+	 * Note: if two requests come in concurrently, we might let them
+	 * both succeed, when one of them should fail.  Not a huge deal.
+	 *
+	 * XXX The limit should be adjusted dynamically to keep the time
+	 * to sync a dataset fixed (around 1-5 seconds?).
+	 */
+
+	if (tempreserve + arc_tempreserve + arc_anon->arcs_size > arc_c / 2 &&
+	    arc_tempreserve + arc_anon->arcs_size > arc_c / 4) {
+		dprintf("failing, arc_tempreserve=%lluK anon=%lluK "
+		    "tempreserve=%lluK arc_c=%lluK\n",
+		    arc_tempreserve>>10, arc_anon->arcs_lsize>>10,
+		    tempreserve>>10, arc_c>>10);
+		return (ERESTART);
+	}
+	atomic_add_64(&arc_tempreserve, tempreserve);
+	return (0);
+}
+
+#ifdef _KERNEL
+static eventhandler_tag zfs_event_lowmem = NULL;
+
+static void
+zfs_lowmem(void *arg __unused, int howto __unused)
+{
+
+	zfs_needfree = 1;
+	cv_signal(&arc_reclaim_thr_cv);
+	while (zfs_needfree)
+		tsleep(&zfs_needfree, 0, "zfs:lowmem", hz / 5);
+}
+#endif
+
+void
+arc_init(void)
+{
+	mutex_init(&arc_reclaim_thr_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&arc_reclaim_thr_cv, NULL, CV_DEFAULT, NULL);
+
+	/* Convert seconds to clock ticks */
+	arc_min_prefetch_lifespan = 1 * hz;
+
+	/* Start out with 1/8 of all memory */
+	arc_c = physmem * PAGESIZE / 8;
+#if 0
+#ifdef _KERNEL
+	/*
+	 * On architectures where the physical memory can be larger
+	 * than the addressable space (intel in 32-bit mode), we may
+	 * need to limit the cache to 1/8 of VM size.
+	 */
+	arc_c = MIN(arc_c, vmem_size(heap_arena, VMEM_ALLOC | VMEM_FREE) / 8);
+#endif
+#endif
+	/* set min cache to 1/32 of all memory, or 64MB, whichever is more */
+	arc_c_min = MAX(arc_c / 4, 64<<20);
+	/* set max to 3/4 of all memory, or all but 1GB, whichever is more */
+	if (arc_c * 8 >= 1<<30)
+		arc_c_max = (arc_c * 8) - (1<<30);
+	else
+		arc_c_max = arc_c_min;
+	arc_c_max = MAX(arc_c * 6, arc_c_max);
+#ifdef notyet
+	/*
+	 * Allow the tunables to override our calculations if they are
+	 * reasonable (ie. over 64MB)
+	 */
+	if (zfs_arc_max > 64<<20 && zfs_arc_max < physmem * PAGESIZE)
+		arc_c_max = zfs_arc_max;
+	if (zfs_arc_min > 64<<20 && zfs_arc_min <= arc_c_max)
+		arc_c_min = zfs_arc_min;
+#endif
+	arc_c = arc_c_max;
+	arc_p = (arc_c >> 1);
+
+	/* if kmem_flags are set, lets try to use less memory */
+	if (kmem_debugging())
+		arc_c = arc_c / 2;
+	if (arc_c < arc_c_min)
+		arc_c = arc_c_min;
+
+	arc_anon = &ARC_anon;
+	arc_mru = &ARC_mru;
+	arc_mru_ghost = &ARC_mru_ghost;
+	arc_mfu = &ARC_mfu;
+	arc_mfu_ghost = &ARC_mfu_ghost;
+	arc_size = 0;
+
+	mutex_init(&arc_anon->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&arc_mru->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&arc_mru_ghost->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&arc_mfu->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&arc_mfu_ghost->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
+
+	list_create(&arc_mru->arcs_list, sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_arc_node));
+	list_create(&arc_mru_ghost->arcs_list, sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_arc_node));
+	list_create(&arc_mfu->arcs_list, sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_arc_node));
+	list_create(&arc_mfu_ghost->arcs_list, sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_arc_node));
+
+	buf_init();
+
+	arc_thread_exit = 0;
+	arc_eviction_list = NULL;
+	mutex_init(&arc_eviction_mtx, NULL, MUTEX_DEFAULT, NULL);
+	bzero(&arc_eviction_hdr, sizeof (arc_buf_hdr_t));
+
+	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
+	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
+
+	if (arc_ksp != NULL) {
+		arc_ksp->ks_data = &arc_stats;
+		kstat_install(arc_ksp);
+	}
+
+	(void) thread_create(NULL, 0, arc_reclaim_thread, NULL, 0, &p0,
+	    TS_RUN, minclsyspri);
+
+#ifdef _KERNEL
+	zfs_event_lowmem = EVENTHANDLER_REGISTER(vm_lowmem, zfs_lowmem, NULL,
+	    EVENTHANDLER_PRI_FIRST);
+#endif
+
+	arc_dead = FALSE;
+}
+
+void
+arc_fini(void)
+{
+	mutex_enter(&arc_reclaim_thr_lock);
+	arc_thread_exit = 1;
+	cv_signal(&arc_reclaim_thr_cv);
+	while (arc_thread_exit != 0)
+		cv_wait(&arc_reclaim_thr_cv, &arc_reclaim_thr_lock);
+	mutex_exit(&arc_reclaim_thr_lock);
+
+	arc_flush();
+
+	arc_dead = TRUE;
+
+	if (arc_ksp != NULL) {
+		kstat_delete(arc_ksp);
+		arc_ksp = NULL;
+	}
+
+	mutex_destroy(&arc_eviction_mtx);
+	mutex_destroy(&arc_reclaim_thr_lock);
+	cv_destroy(&arc_reclaim_thr_cv);
+
+	list_destroy(&arc_mru->arcs_list);
+	list_destroy(&arc_mru_ghost->arcs_list);
+	list_destroy(&arc_mfu->arcs_list);
+	list_destroy(&arc_mfu_ghost->arcs_list);
+
+	mutex_destroy(&arc_anon->arcs_mtx);
+	mutex_destroy(&arc_mru->arcs_mtx);
+	mutex_destroy(&arc_mru_ghost->arcs_mtx);
+	mutex_destroy(&arc_mfu->arcs_mtx);
+	mutex_destroy(&arc_mfu_ghost->arcs_mtx);
+
+	buf_fini();
+
+#ifdef _KERNEL
+	if (zfs_event_lowmem != NULL)
+		EVENTHANDLER_DEREGISTER(vm_lowmem, zfs_event_lowmem);
+#endif
+}