BTRFS ===== Btrfs is a copy on write filesystem for Linux aimed at implementing advanced features while focusing on fault tolerance, repair and easy administration. Initially developed by Oracle, Btrfs is licensed under the GPL and open for contribution from anyone. Linux has a wealth of filesystems to choose from, but we are facing a number of challenges with scaling to the large storage subsystems that are becoming common in today's data centers. Filesystems need to scale in their ability to address and manage large storage, and also in their ability to detect, repair and tolerate errors in the data stored on disk. Btrfs is under heavy development, and is not suitable for any uses other than benchmarking and review. The Btrfs disk format is not yet finalized. The main Btrfs features include: * Extent based file storage (2^64 max file size) * Space efficient packing of small files * Space efficient indexed directories * Dynamic inode allocation * Writable snapshots * Subvolumes (separate internal filesystem roots) * Object level mirroring and striping * Checksums on data and metadata (multiple algorithms available) * Compression * Integrated multiple device support, with several raid algorithms * Online filesystem check (not yet implemented) * Very fast offline filesystem check * Efficient incremental backup and FS mirroring (not yet implemented) * Online filesystem defragmentation Mount Options ============= When mounting a btrfs filesystem, the following option are accepted. Options with (*) are default options and will not show in the mount options. alloc_start= Debugging option to force all block allocations above a certain byte threshold on each block device. The value is specified in bytes, optionally with a K, M, or G suffix, case insensitive. Default is 1MB. noautodefrag(*) autodefrag Disable/enable auto defragmentation. Auto defragmentation detects small random writes into files and queue them up for the defrag process. Works best for small files; Not well suited for large database workloads. check_int check_int_data check_int_print_mask= These debugging options control the behavior of the integrity checking module (the BTRFS_FS_CHECK_INTEGRITY config option required). check_int enables the integrity checker module, which examines all block write requests to ensure on-disk consistency, at a large memory and CPU cost. check_int_data includes extent data in the integrity checks, and implies the check_int option. check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values as defined in fs/btrfs/check-integrity.c, to control the integrity checker module behavior. See comments at the top of fs/btrfs/check-integrity.c for more info. commit= Set the interval of periodic commit, 30 seconds by default. Higher values defer data being synced to permanent storage with obvious consequences when the system crashes. The upper bound is not forced, but a warning is printed if it's more than 300 seconds (5 minutes). compress compress= compress-force compress-force= Control BTRFS file data compression. Type may be specified as "zlib" "lzo" or "no" (for no compression, used for remounting). If no type is specified, zlib is used. If compress-force is specified, all files will be compressed, whether or not they compress well. If compression is enabled, nodatacow and nodatasum are disabled. degraded Allow mounts to continue with missing devices. A read-write mount may fail with too many devices missing, for example if a stripe member is completely missing. device= Specify a device during mount so that ioctls on the control device can be avoided. Especially useful when trying to mount a multi-device setup as root. May be specified multiple times for multiple devices. nodiscard(*) discard Disable/enable discard mount option. Discard issues frequent commands to let the block device reclaim space freed by the filesystem. This is useful for SSD devices, thinly provisioned LUNs and virtual machine images, but may have a significant performance impact. (The fstrim command is also available to initiate batch trims from userspace). noenospc_debug(*) enospc_debug Disable/enable debugging option to be more verbose in some ENOSPC conditions. fatal_errors= Action to take when encountering a fatal error: "bug" - BUG() on a fatal error. This is the default. "panic" - panic() on a fatal error. noflushoncommit(*) flushoncommit The 'flushoncommit' mount option forces any data dirtied by a write in a prior transaction to commit as part of the current commit. This makes the committed state a fully consistent view of the file system from the application's perspective (i.e., it includes all completed file system operations). This was previously the behavior only when a snapshot is created. inode_cache Enable free inode number caching. Defaults to off due to an overflow problem when the free space crcs don't fit inside a single page. max_inline= Specify the maximum amount of space, in bytes, that can be inlined in a metadata B-tree leaf. The value is specified in bytes, optionally with a K, M, or G suffix, case insensitive. In practice, this value is limited by the root sector size, with some space unavailable due to leaf headers. For a 4k sector size, max inline data is ~3900 bytes. metadata_ratio= Specify that 1 metadata chunk should be allocated after every data chunks. Off by default. acl(*) noacl Enable/disable support for Posix Access Control Lists (ACLs). See the acl(5) manual page for more information about ACLs. barrier(*) nobarrier Enable/disable the use of block layer write barriers. Write barriers ensure that certain IOs make it through the device cache and are on persistent storage. If disabled on a device with a volatile (non-battery-backed) write-back cache, nobarrier option will lead to filesystem corruption on a system crash or power loss. datacow(*) nodatacow Enable/disable data copy-on-write for newly created files. Nodatacow implies nodatasum, and disables all compression. datasum(*) nodatasum Enable/disable data checksumming for newly created files. Datasum implies datacow. treelog(*) notreelog Enable/disable the tree logging used for fsync and O_SYNC writes. nologreplay Disable the log tree replay at mount time to prevent filesystem from getting modified. Must be used with 'ro' mount option. A filesystem mounted with this option cannot transition to a read-write mount via remount,rw - the filesystem must be unmounted and mounted back again if read-write access is desired. usebackuproot Enable attempts to use backup tree roots if a bad tree root is found at mount time. Currently this scans a list of 4 previous tree roots and tries to use the first readable. And since the mount option doesn't affect any behavior after mount, it won't be shown in mount info. Prior to 4.6, this was done by 'recovery' option that has been deprecated, but will work. rescan_uuid_tree Force check and rebuild procedure of the UUID tree. This should not normally be needed. skip_balance Skip automatic resume of interrupted balance operation after mount. May be resumed with "btrfs balance resume." space_cache (*) Enable the on-disk freespace cache. nospace_cache Disable freespace cache loading without clearing the cache. clear_cache Force clearing and rebuilding of the disk space cache if something has gone wrong. ssd nossd ssd_spread Options to control ssd allocation schemes. By default, BTRFS will enable or disable ssd allocation heuristics depending on whether a rotational or non-rotational disk is in use. The ssd and nossd options can override this autodetection. The ssd_spread mount option attempts to allocate into big chunks of unused space, and may perform better on low-end ssds. ssd_spread implies ssd, enabling all other ssd heuristics as well. subvol= Mount subvolume at rather than the root subvolume. is relative to the top level subvolume. subvolid= Mount subvolume specified by an ID number rather than the root subvolume. This allows mounting of subvolumes which are not in the root of the mounted filesystem. You can use "btrfs subvolume list" to see subvolume ID numbers. subvolrootid= (deprecated) Mount subvolume specified by rather than the root subvolume. This allows mounting of subvolumes which are not in the root of the mounted filesystem. You can use "btrfs subvolume show " to see the object ID for a subvolume. thread_pool= The number of worker threads to allocate. The default number is equal to the number of CPUs + 2, or 8, whichever is smaller. user_subvol_rm_allowed Allow subvolumes to be deleted by a non-root user. Use with caution. MAILING LIST ============ There is a Btrfs mailing list hosted on vger.kernel.org. You can find details on how to subscribe here: http://vger.kernel.org/vger-lists.html#linux-btrfs Mailing list archives are available from gmane: http://dir.gmane.org/gmane.comp.file-systems.btrfs IRC === Discussion of Btrfs also occurs on the #btrfs channel of the Freenode IRC network. UTILITIES ========= Userspace tools for creating and manipulating Btrfs file systems are available from the git repository at the following location: http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs.git These include the following tools: * mkfs.btrfs: create a filesystem * btrfs: a single tool to manage the filesystems, refer to the manpage for more details * 'btrfsck' or 'btrfs check': do a consistency check of the filesystem Other tools for specific tasks: * btrfs-convert: in-place conversion from ext2/3/4 filesystems * btrfs-image: dump filesystem metadata for debugging