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+
+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=<bytes>
+ 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=<value>
+ 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=<seconds>
+ 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=<type>
+ compress-force
+ compress-force=<type>
+ 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=<devicepath>
+ 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>
+ 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=<bytes>
+ 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 sectorsize, max inline data is ~3900 bytes.
+
+ metadata_ratio=<value>
+ Specify that 1 metadata chunk should be allocated after every <value>
+ 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.
+
+ recovery
+ Enable autorecovery attempts if a bad tree root is found at mount time.
+ Currently this scans a list of several previous tree roots and tries to
+ use the first readable.
+
+ 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 nonrotational 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=<path>
+ Mount subvolume at <path> rather than the root subvolume. <path> is
+ relative to the top level subvolume.
+
+ subvolid=<ID>
+ 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=<objectid> (deprecated)
+ Mount subvolume specified by <objectid> 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=<number>
+ 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