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Documentation / filesystems / btrfs.txt

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Based on kernel version 4.3. Page generated on 2015-11-02 12:49 EST.

3	=====
5	Btrfs is a copy on write filesystem for Linux aimed at
6	implementing advanced features while focusing on fault tolerance,
7	repair and easy administration. Initially developed by Oracle, Btrfs
8	is licensed under the GPL and open for contribution from anyone.
10	Linux has a wealth of filesystems to choose from, but we are facing a
11	number of challenges with scaling to the large storage subsystems that
12	are becoming common in today's data centers. Filesystems need to scale
13	in their ability to address and manage large storage, and also in
14	their ability to detect, repair and tolerate errors in the data stored
15	on disk.  Btrfs is under heavy development, and is not suitable for
16	any uses other than benchmarking and review. The Btrfs disk format is
17	not yet finalized.
19	The main Btrfs features include:
21	    * Extent based file storage (2^64 max file size)
22	    * Space efficient packing of small files
23	    * Space efficient indexed directories
24	    * Dynamic inode allocation
25	    * Writable snapshots
26	    * Subvolumes (separate internal filesystem roots)
27	    * Object level mirroring and striping
28	    * Checksums on data and metadata (multiple algorithms available)
29	    * Compression
30	    * Integrated multiple device support, with several raid algorithms
31	    * Online filesystem check (not yet implemented)
32	    * Very fast offline filesystem check
33	    * Efficient incremental backup and FS mirroring (not yet implemented)
34	    * Online filesystem defragmentation
37	Mount Options
38	=============
40	When mounting a btrfs filesystem, the following option are accepted.
41	Options with (*) are default options and will not show in the mount options.
43	  alloc_start=<bytes>
44		Debugging option to force all block allocations above a certain
45		byte threshold on each block device.  The value is specified in
46		bytes, optionally with a K, M, or G suffix, case insensitive.
47		Default is 1MB.
49	  noautodefrag(*)
50	  autodefrag
51		Disable/enable auto defragmentation.
52		Auto defragmentation detects small random writes into files and queue
53		them up for the defrag process.  Works best for small files;
54		Not well suited for large database workloads.
56	  check_int
57	  check_int_data
58	  check_int_print_mask=<value>
59		These debugging options control the behavior of the integrity checking
60		module (the BTRFS_FS_CHECK_INTEGRITY config option required).
62		check_int enables the integrity checker module, which examines all
63		block write requests to ensure on-disk consistency, at a large
64		memory and CPU cost.
66		check_int_data includes extent data in the integrity checks, and
67		implies the check_int option.
69		check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values
70		as defined in fs/btrfs/check-integrity.c, to control the integrity
71		checker module behavior.
73		See comments at the top of fs/btrfs/check-integrity.c for more info.
75	  commit=<seconds>
76		Set the interval of periodic commit, 30 seconds by default. Higher
77		values defer data being synced to permanent storage with obvious
78		consequences when the system crashes. The upper bound is not forced,
79		but a warning is printed if it's more than 300 seconds (5 minutes).
81	  compress
82	  compress=<type>
83	  compress-force
84	  compress-force=<type>
85		Control BTRFS file data compression.  Type may be specified as "zlib"
86		"lzo" or "no" (for no compression, used for remounting).  If no type
87		is specified, zlib is used.  If compress-force is specified,
88		all files will be compressed, whether or not they compress well.
89		If compression is enabled, nodatacow and nodatasum are disabled.
91	  degraded
92		Allow mounts to continue with missing devices.  A read-write mount may
93		fail with too many devices missing, for example if a stripe member
94		is completely missing.
96	  device=<devicepath>
97		Specify a device during mount so that ioctls on the control device
98		can be avoided.  Especially useful when trying to mount a multi-device
99		setup as root.  May be specified multiple times for multiple devices.
101	  nodiscard(*)
102	  discard
103		Disable/enable discard mount option.
104		Discard issues frequent commands to let the block device reclaim space
105		freed by the filesystem.
106		This is useful for SSD devices, thinly provisioned
107		LUNs and virtual machine images, but may have a significant
108		performance impact.  (The fstrim command is also available to
109		initiate batch trims from userspace).
111	  noenospc_debug(*)
112	  enospc_debug
113		Disable/enable debugging option to be more verbose in some ENOSPC conditions.
115	  fatal_errors=<action>
116		Action to take when encountering a fatal error:
117		  "bug" - BUG() on a fatal error.  This is the default.
118		  "panic" - panic() on a fatal error.
120	  noflushoncommit(*)
121	  flushoncommit
122		The 'flushoncommit' mount option forces any data dirtied by a write in a
123		prior transaction to commit as part of the current commit.  This makes
124		the committed state a fully consistent view of the file system from the
125		application's perspective (i.e., it includes all completed file system
126		operations).  This was previously the behavior only when a snapshot is
127		created.
129	  inode_cache
130		Enable free inode number caching.   Defaults to off due to an overflow
131		problem when the free space crcs don't fit inside a single page.
133	  max_inline=<bytes>
134		Specify the maximum amount of space, in bytes, that can be inlined in
135		a metadata B-tree leaf.  The value is specified in bytes, optionally
136		with a K, M, or G suffix, case insensitive.  In practice, this value
137		is limited by the root sector size, with some space unavailable due
138		to leaf headers.  For a 4k sector size, max inline data is ~3900 bytes.
140	  metadata_ratio=<value>
141		Specify that 1 metadata chunk should be allocated after every <value>
142		data chunks.  Off by default.
144	  acl(*)
145	  noacl
146		Enable/disable support for Posix Access Control Lists (ACLs).  See the
147		acl(5) manual page for more information about ACLs.
149	  barrier(*)
150	  nobarrier
151	        Enable/disable the use of block layer write barriers.  Write barriers
152		ensure that certain IOs make it through the device cache and are on
153		persistent storage. If disabled on a device with a volatile
154		(non-battery-backed) write-back cache, nobarrier option will lead to
155		filesystem corruption on a system crash or power loss.
157	  datacow(*)
158	  nodatacow
159		Enable/disable data copy-on-write for newly created files.
160		Nodatacow implies nodatasum, and disables all compression.
162	  datasum(*)
163	  nodatasum
164		Enable/disable data checksumming for newly created files.
165		Datasum implies datacow.
167	  treelog(*)
168	  notreelog
169		Enable/disable the tree logging used for fsync and O_SYNC writes.
171	  recovery
172		Enable autorecovery attempts if a bad tree root is found at mount time.
173		Currently this scans a list of several previous tree roots and tries to
174		use the first readable.
176	  rescan_uuid_tree
177		Force check and rebuild procedure of the UUID tree. This should not
178		normally be needed.
180	  skip_balance
181		Skip automatic resume of interrupted balance operation after mount.
182		May be resumed with "btrfs balance resume."
184	  space_cache (*)
185		Enable the on-disk freespace cache.
186	  nospace_cache
187		Disable freespace cache loading without clearing the cache.
188	  clear_cache
189		Force clearing and rebuilding of the disk space cache if something
190		has gone wrong.
192	  ssd
193	  nossd
194	  ssd_spread
195		Options to control ssd allocation schemes.  By default, BTRFS will
196		enable or disable ssd allocation heuristics depending on whether a
197		rotational or non-rotational disk is in use.  The ssd and nossd options
198		can override this autodetection.
200		The ssd_spread mount option attempts to allocate into big chunks
201		of unused space, and may perform better on low-end ssds.  ssd_spread
202		implies ssd, enabling all other ssd heuristics as well.
204	  subvol=<path>
205		Mount subvolume at <path> rather than the root subvolume.  <path> is
206		relative to the top level subvolume.
208	  subvolid=<ID>
209		Mount subvolume specified by an ID number rather than the root subvolume.
210		This allows mounting of subvolumes which are not in the root of the mounted
211		filesystem.
212		You can use "btrfs subvolume list" to see subvolume ID numbers.
214	  subvolrootid=<objectid> (deprecated)
215		Mount subvolume specified by <objectid> rather than the root subvolume.
216		This allows mounting of subvolumes which are not in the root of the mounted
217		filesystem.
218		You can use "btrfs subvolume show " to see the object ID for a subvolume.
220	  thread_pool=<number>
221		The number of worker threads to allocate.  The default number is equal
222		to the number of CPUs + 2, or 8, whichever is smaller.
224	  user_subvol_rm_allowed
225		Allow subvolumes to be deleted by a non-root user. Use with caution.
228	============
230	There is a Btrfs mailing list hosted on vger.kernel.org. You can
231	find details on how to subscribe here:
233	http://vger.kernel.org/vger-lists.html#linux-btrfs
235	Mailing list archives are available from gmane:
237	http://dir.gmane.org/gmane.comp.file-systems.btrfs
241	IRC
242	===
244	Discussion of Btrfs also occurs on the #btrfs channel of the Freenode
245	IRC network.
250		=========
252	Userspace tools for creating and manipulating Btrfs file systems are
253	available from the git repository at the following location:
255	 http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git
256	 git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs.git
258	These include the following tools:
260	* mkfs.btrfs: create a filesystem
262	* btrfs: a single tool to manage the filesystems, refer to the manpage for more details
264	* 'btrfsck' or 'btrfs check': do a consistency check of the filesystem
266	Other tools for specific tasks:
268	* btrfs-convert: in-place conversion from ext2/3/4 filesystems
270	* btrfs-image: dump filesystem metadata for debugging
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