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
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
26 * Subvolumes (separate internal filesystem roots)
27 * Object level mirroring and striping
28 * Checksums on data and metadata (multiple algorithms available)
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
40 When mounting a btrfs filesystem, the following option are accepted.
41 Unless otherwise specified, all options default to off.
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.
50 Detect small random writes into files and queue them up for the
51 defrag process. Works best for small files; Not well suited for
52 large database workloads.
56 check_int_print_mask=<value>
57 These debugging options control the behavior of the integrity checking
58 module (the BTRFS_FS_CHECK_INTEGRITY config option required).
60 check_int enables the integrity checker module, which examines all
61 block write requests to ensure on-disk consistency, at a large
64 check_int_data includes extent data in the integrity checks, and
65 implies the check_int option.
67 check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values
68 as defined in fs/btrfs/check-integrity.c, to control the integrity
69 checker module behavior.
71 See comments at the top of fs/btrfs/check-integrity.c for more info.
74 Set the interval of periodic commit, 30 seconds by default. Higher
75 values defer data being synced to permanent storage with obvious
76 consequences when the system crashes. The upper bound is not forced,
77 but a warning is printed if it's more than 300 seconds (5 minutes).
83 Control BTRFS file data compression. Type may be specified as "zlib"
84 "lzo" or "no" (for no compression, used for remounting). If no type
85 is specified, zlib is used. If compress-force is specified,
86 all files will be compressed, whether or not they compress well.
87 If compression is enabled, nodatacow and nodatasum are disabled.
90 Allow mounts to continue with missing devices. A read-write mount may
91 fail with too many devices missing, for example if a stripe member
92 is completely missing.
95 Specify a device during mount so that ioctls on the control device
96 can be avoided. Especially useful when trying to mount a multi-device
97 setup as root. May be specified multiple times for multiple devices.
100 Issue frequent commands to let the block device reclaim space freed by
101 the filesystem. This is useful for SSD devices, thinly provisioned
102 LUNs and virtual machine images, but may have a significant
103 performance impact. (The fstrim command is also available to
104 initiate batch trims from userspace).
107 Debugging option to be more verbose in some ENOSPC conditions.
109 fatal_errors=<action>
110 Action to take when encountering a fatal error:
111 "bug" - BUG() on a fatal error. This is the default.
112 "panic" - panic() on a fatal error.
115 The 'flushoncommit' mount option forces any data dirtied by a write in a
116 prior transaction to commit as part of the current commit. This makes
117 the committed state a fully consistent view of the file system from the
118 application's perspective (i.e., it includes all completed file system
119 operations). This was previously the behavior only when a snapshot is
123 Enable free inode number caching. Defaults to off due to an overflow
124 problem when the free space crcs don't fit inside a single page.
127 Specify the maximum amount of space, in bytes, that can be inlined in
128 a metadata B-tree leaf. The value is specified in bytes, optionally
129 with a K, M, or G suffix, case insensitive. In practice, this value
130 is limited by the root sector size, with some space unavailable due
131 to leaf headers. For a 4k sectorsize, max inline data is ~3900 bytes.
133 metadata_ratio=<value>
134 Specify that 1 metadata chunk should be allocated after every <value>
135 data chunks. Off by default.
138 Disable support for Posix Access Control Lists (ACLs). See the
139 acl(5) manual page for more information about ACLs.
142 Disables the use of block layer write barriers. Write barriers ensure
143 that certain IOs make it through the device cache and are on persistent
144 storage. If used on a device with a volatile (non-battery-backed)
145 write-back cache, this option will lead to filesystem corruption on a
146 system crash or power loss.
149 Disable data copy-on-write for newly created files. Implies nodatasum,
150 and disables all compression.
153 Disable data checksumming for newly created files.
156 Disable the tree logging used for fsync and O_SYNC writes.
159 Enable autorecovery attempts if a bad tree root is found at mount time.
160 Currently this scans a list of several previous tree roots and tries to
161 use the first readable.
164 Force check and rebuild procedure of the UUID tree. This should not
168 Skip automatic resume of interrupted balance operation after mount.
169 May be resumed with "btrfs balance resume."
172 Enable the on-disk freespace cache.
174 Disable freespace cache loading without clearing the cache.
176 Force clearing and rebuilding of the disk space cache if something
182 Options to control ssd allocation schemes. By default, BTRFS will
183 enable or disable ssd allocation heuristics depending on whether a
184 rotational or nonrotational disk is in use. The ssd and nossd options
185 can override this autodetection.
187 The ssd_spread mount option attempts to allocate into big chunks
188 of unused space, and may perform better on low-end ssds. ssd_spread
189 implies ssd, enabling all other ssd heuristics as well.
192 Mount subvolume at <path> rather than the root subvolume. <path> is
193 relative to the top level subvolume.
196 Mount subvolume specified by an ID number rather than the root subvolume.
197 This allows mounting of subvolumes which are not in the root of the mounted
199 You can use "btrfs subvolume list" to see subvolume ID numbers.
201 subvolrootid=<objectid> (deprecated)
202 Mount subvolume specified by <objectid> rather than the root subvolume.
203 This allows mounting of subvolumes which are not in the root of the mounted
205 You can use "btrfs subvolume show " to see the object ID for a subvolume.
208 The number of worker threads to allocate. The default number is equal
209 to the number of CPUs + 2, or 8, whichever is smaller.
211 user_subvol_rm_allowed
212 Allow subvolumes to be deleted by a non-root user. Use with caution.
217 There is a Btrfs mailing list hosted on vger.kernel.org. You can
218 find details on how to subscribe here:
220 http://vger.kernel.org/vger-lists.html#linux-btrfs
222 Mailing list archives are available from gmane:
224 http://dir.gmane.org/gmane.comp.file-systems.btrfs
231 Discussion of Btrfs also occurs on the #btrfs channel of the Freenode
239 Userspace tools for creating and manipulating Btrfs file systems are
240 available from the git repository at the following location:
242 http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git
243 git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs.git
245 These include the following tools:
247 * mkfs.btrfs: create a filesystem
249 * btrfs: a single tool to manage the filesystems, refer to the manpage for more details
251 * 'btrfsck' or 'btrfs check': do a consistency check of the filesystem
253 Other tools for specific tasks:
255 * btrfs-convert: in-place conversion from ext2/3/4 filesystems
257 * btrfs-image: dump filesystem metadata for debugging