Add a 3rd entry in the cache, which keeps the end position

from just before extending a file.  This has the desired effect
of keeping the write speed constant.  And yes, that helps a lot
copying large files always at full speed now, and I have seen
improvements using benchmarks/bonnie.

Stolen from:	NetBSD
Reviewed by:	bde

2 files changed, 19 insertions, 3 deletions

diff --git a/sys/fs/msdosfs/denode.h b/sys/fs/msdosfs/denode.h
index 4fa51aa..3aac0bc 100644
--- a/sys/fs/msdosfs/denode.h
+++ b/sys/fs/msdosfs/denode.h
@@ -116,10 +116,12 @@ struct fatcache {
  * cache is probably pretty worthless if a file is opened by multiple
  * processes.
  */
-#define	FC_SIZE		2	/* number of entries in the cache */
+#define	FC_SIZE		3	/* number of entries in the cache */
 #define	FC_LASTMAP	0	/* entry the last call to pcbmap() resolved
 				 * to */
 #define	FC_LASTFC	1	/* entry for the last cluster in the file */
+#define	FC_NEXTTOLASTFC	2	/* entry for a close to the last cluster in
+				 * the file */
 
 #define	FCE_EMPTY	0xffffffff	/* doesn't represent an actual cluster # */
 
@@ -130,6 +132,13 @@ struct fatcache {
 	(dep)->de_fc[(slot)].fc_frcn = (frcn); \
 	(dep)->de_fc[(slot)].fc_fsrcn = (fsrcn);
 
+#define	fc_last_to_nexttolast(dep) do {		 \
+	(dep)->de_fc[FC_NEXTTOLASTFC].fc_frcn =  \
+	(dep)->de_fc[FC_LASTFC].fc_frcn;	 \
+	(dep)->de_fc[FC_NEXTTOLASTFC].fc_fsrcn = \
+	(dep)->de_fc[FC_LASTFC].fc_fsrcn;	 \
+} while (0)
+
 /*
  * This is the in memory variant of a dos directory entry.  It is usually
  * contained within a vnode.
diff --git a/sys/fs/msdosfs/msdosfs_fat.c b/sys/fs/msdosfs/msdosfs_fat.c
index 31cf977..1d57a9c 100644
--- a/sys/fs/msdosfs/msdosfs_fat.c
+++ b/sys/fs/msdosfs/msdosfs_fat.c
@@ -79,6 +79,8 @@ static int fc_bmapcalls;		/* # of times pcbmap was called		 */
 static int fc_lmdistance[LMMAX];/* counters for how far off the last
 				 * cluster mapped entry was. */
 static int fc_largedistance;	/* off by more than LMMAX		 */
+static int fc_wherefrom, fc_whereto, fc_lastclust;
+static int pm_fatblocksize;
 
 static int	chainalloc(struct msdosfsmount *pmp, u_long start,
 		    u_long count, u_long fillwith, u_long *retcluster,
@@ -119,6 +121,7 @@ fatblock(pmp, ofs, bnp, sizep, bop)
 		*sizep = size;
 	if (bop)
 		*bop = ofs % pmp->pm_fatblocksize;
+		pm_fatblocksize =  pmp->pm_fatblocksize;
 }
 
 /*
@@ -210,9 +213,12 @@ pcbmap(dep, findcn, bnp, cnp, sp)
 	 */
 	i = 0;
 	fc_lookup(dep, findcn, &i, &cn);
-	if ((bn = findcn - i) >= LMMAX)
+	if ((bn = findcn - i) >= LMMAX) {
 		fc_largedistance++;
-	else
+		fc_wherefrom = i;
+		fc_whereto = findcn;
+		fc_lastclust = dep->de_fc[FC_LASTFC].fc_frcn;
+	} else
 		fc_lmdistance[bn]++;
 
 	/*
@@ -1025,6 +1031,7 @@ extendfile(dep, count, bpp, ncp, flags)
 			return (error);
 	}
 
+	fc_last_to_nexttolast(dep);
 	while (count > 0) {
 		/*
 		 * Allocate a new cluster chain and cat onto the end of the