Administration

The main administration tool for BTRFS filesystems is btrfs(8). Please refer to the manual pages of the subcommands for further documentation.

Mount options

This section describes mount options specific to BTRFS. For the generic mount options please refer to mount(8) manpage. The options are sorted alphabetically (discarding the no prefix).

Note

Most mount options apply to the whole filesystem and only options in the first mounted subvolume will take effect. This is due to lack of implementation and may change in the future. This means that (for example) you can’t set per-subvolume nodatacow, nodatasum, or compress using mount options. This should eventually be fixed, but it has proved to be difficult to implement correctly within the Linux VFS framework.

Mount options are processed in order, only the last occurrence of an option takes effect and may disable other options due to constraints (see eg. nodatacow and compress). The output of mount command shows which options have been applied.

acl, noacl

(default: on)

Enable/disable support for Posix Access Control Lists (ACLs). See the acl(5) manual page for more information about ACLs.

The support for ACL is build-time configurable (BTRFS_FS_POSIX_ACL) and mount fails if acl is requested but the feature is not compiled in.

autodefrag, noautodefrag

(since: 3.0, default: off)

Enable automatic file defragmentation. When enabled, small random writes into files (in a range of tens of kilobytes, currently it’s 64KiB) are detected and queued up for the defragmentation process. Not well suited for large database workloads.

The read latency may increase due to reading the adjacent blocks that make up the range for defragmentation, successive write will merge the blocks in the new location.

Warning

Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2 as well as with Linux stable kernel versions ≥ 3.10.31, ≥ 3.12.12 or ≥ 3.13.4 will break up the reflinks of COW data (for example files copied with cp --reflink, snapshots or de-duplicated data). This may cause considerable increase of space usage depending on the broken up reflinks.

barrier, nobarrier

(default: on)

Ensure that all IO write operations make it through the device cache and are stored permanently when the filesystem is at its consistency checkpoint. This typically means that a flush command is sent to the device that will synchronize all pending data and ordinary metadata blocks, then writes the superblock and issues another flush.

The write flushes incur a slight hit and also prevent the IO block scheduler to reorder requests in a more effective way. Disabling barriers gets rid of that penalty but will most certainly lead to a corrupted filesystem in case of a crash or power loss. The ordinary metadata blocks could be yet unwritten at the time the new superblock is stored permanently, expecting that the block pointers to metadata were stored permanently before.

On a device with a volatile battery-backed write-back cache, the nobarrier option will not lead to filesystem corruption as the pending blocks are supposed to make it to the permanent storage.

check_int, check_int_data, check_int_print_mask=<value>

(since: 3.0, default: off)

These debugging options control the behavior of the integrity checking module (the BTRFS_FS_CHECK_INTEGRITY config option required). The main goal is to verify that all blocks from a given transaction period are properly linked.

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 information.

clear_cache

Force clearing and rebuilding of the disk space cache if something has gone wrong. See also: space_cache.

commit=<seconds>

(since: 3.12, default: 30)

Set the interval of periodic transaction commit when data are synchronized to permanent storage. Higher interval values lead to larger amount of unwritten data, which has 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). Use with care.

compress, compress=<type[:level]>, compress-force, compress-force=<type[:level]>

(default: off, level support since: 5.1)

Control BTRFS file data compression. Type may be specified as zlib, lzo, zstd or no (for no compression, used for remounting). If no type is specified, zlib is used. If compress-force is specified, then compression will always be attempted, but the data may end up uncompressed if the compression would make them larger.

Both zlib and zstd (since version 5.1) expose the compression level as a tunable knob with higher levels trading speed and memory (zstd) for higher compression ratios. This can be set by appending a colon and the desired level. Zlib accepts the range [1, 9] and zstd accepts [1, 15]. If no level is set, both currently use a default level of 3. The value 0 is an alias for the default level.

Otherwise some simple heuristics are applied to detect an incompressible file. If the first blocks written to a file are not compressible, the whole file is permanently marked to skip compression. As this is too simple, the compress-force is a workaround that will compress most of the files at the cost of some wasted CPU cycles on failed attempts. Since kernel 4.15, a set of heuristic algorithms have been improved by using frequency sampling, repeated pattern detection and Shannon entropy calculation to avoid that.

Note

If compression is enabled, nodatacow and nodatasum are disabled.

datacow, nodatacow

(default: on)

Enable data copy-on-write for newly created files. Nodatacow implies nodatasum, and disables compression. All files created under nodatacow are also set the NOCOW file attribute (see chattr(1)).

Note

If nodatacow or nodatasum are enabled, compression is disabled.

Updates in-place improve performance for workloads that do frequent overwrites, at the cost of potential partial writes, in case the write is interrupted (system crash, device failure).

datasum, nodatasum

(default: on)

Enable data checksumming for newly created files. Datasum implies datacow, ie. the normal mode of operation. All files created under nodatasum inherit the “no checksums” property, however there’s no corresponding file attribute (see chattr(1)).

Note

If nodatacow or nodatasum are enabled, compression is disabled.

There is a slight performance gain when checksums are turned off, the corresponding metadata blocks holding the checksums do not need to updated. The cost of checksumming of the blocks in memory is much lower than the IO, modern CPUs feature hardware support of the checksumming algorithm.

degraded

(default: off)

Allow mounts with less devices than the RAID profile constraints require. A read-write mount (or remount) may fail when there are too many devices missing, for example if a stripe member is completely missing from RAID0.

Since 4.14, the constraint checks have been improved and are verified on the chunk level, not an the device level. This allows degraded mounts of filesystems with mixed RAID profiles for data and metadata, even if the device number constraints would not be satisfied for some of the profiles.

Example: metadata -- raid1, data -- single, devices -- /dev/sda, /dev/sdb

Suppose the data are completely stored on sda, then missing sdb will not prevent the mount, even if 1 missing device would normally prevent (any) single profile to mount. In case some of the data chunks are stored on sdb, then the constraint of single/data is not satisfied and the filesystem cannot be mounted.

device=<devicepath>

Specify a path to a device that will be scanned for BTRFS filesystem during mount. This is usually done automatically by a device manager (like udev) or using the btrfs device scan command (eg. run from the initial ramdisk). In cases where this is not possible the device mount option can help.

Note

Booting eg. a RAID1 system may fail even if all filesystem’s device paths are provided as the actual device nodes may not be discovered by the system at that point.

discard, discard=sync, discard=async, nodiscard

(default: off, async support since: 5.6)

Enable discarding of freed file blocks. This is useful for SSD devices, thinly provisioned LUNs, or virtual machine images; however, every storage layer must support discard for it to work.

In the synchronous mode (sync or without option value), lack of asynchronous queued TRIM on the backing device TRIM can severely degrade performance, because a synchronous TRIM operation will be attempted instead. Queued TRIM requires newer than SATA revision 3.1 chipsets and devices.

The asynchronous mode (async) gathers extents in larger chunks before sending them to the devices for TRIM. The overhead and performance impact should be negligible compared to the previous mode and it’s supposed to be the preferred mode if needed.

If it is not necessary to immediately discard freed blocks, then the fstrim tool can be used to discard all free blocks in a batch. Scheduling a TRIM during a period of low system activity will prevent latent interference with the performance of other operations. Also, a device may ignore the TRIM command if the range is too small, so running a batch discard has a greater probability of actually discarding the blocks.

enospc_debug, noenospc_debug

(default: off)

Enable verbose output for some ENOSPC conditions. It’s safe to use but can be noisy if the system reaches near-full state.

fatal_errors=<action>

(since: 3.4, default: bug)

Action to take when encountering a fatal error.

bug

BUG() on a fatal error, the system will stay in the crashed state and may be still partially usable, but reboot is required for full operation

panic

panic() on a fatal error, depending on other system configuration, this may be followed by a reboot. Please refer to the documentation of kernel boot parameters, eg. panic, oops or crashkernel.

flushoncommit, noflushoncommit

(default: off)

This option forces any data dirtied by a write in a prior transaction to commit as part of the current commit, effectively a full filesystem sync.

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 was created.

When off, the filesystem is consistent but buffered writes may last more than one transaction commit.

fragment=<type>

(depends on compile-time option BTRFS_DEBUG, since: 4.4, default: off)

A debugging helper to intentionally fragment given type of block groups. The type can be data, metadata or all. This mount option should not be used outside of debugging environments and is not recognized if the kernel config option BTRFS_DEBUG is not enabled.

nologreplay

(default: off, even read-only)

The tree-log contains pending updates to the filesystem until the full commit. The log is replayed on next mount, this can be disabled by this option. See also treelog. Note that nologreplay is the same as norecovery.

Warning

Currently, the tree log is replayed even with a read-only mount! To disable that behaviour, mount also with nologreplay.

max_inline=<bytes>

(default: min(2048, page size) )

Specify the maximum amount of space, that can be inlined in a metadata b-tree leaf. The value is specified in bytes, optionally with a K suffix (case insensitive). In practice, this value is limited by the filesystem block size (named sectorsize at mkfs time), and memory page size of the system. In case of sectorsize limit, there’s some space unavailable due to leaf headers. For example, a 4KiB sectorsize, maximum size of inline data is about 3900 bytes.

Inlining can be completely turned off by specifying 0. This will increase data block slack if file sizes are much smaller than block size but will reduce metadata consumption in return.

Note

The default value has changed to 2048 in kernel 4.6.

metadata_ratio=<value>

(default: 0, internal logic)

Specifies that 1 metadata chunk should be allocated after every value data chunks. Default behaviour depends on internal logic, some percent of unused metadata space is attempted to be maintained but is not always possible if there’s not enough space left for chunk allocation. The option could be useful to override the internal logic in favor of the metadata allocation if the expected workload is supposed to be metadata intense (snapshots, reflinks, xattrs, inlined files).

norecovery

(since: 4.5, default: off)

Do not attempt any data recovery at mount time. This will disable logreplay and avoids other write operations. Note that this option is the same as nologreplay.

Note

The opposite option recovery used to have different meaning but was changed for consistency with other filesystems, where norecovery is used for skipping log replay. BTRFS does the same and in general will try to avoid any write operations.

rescan_uuid_tree

(since: 3.12, default: off)

Force check and rebuild procedure of the UUID tree. This should not normally be needed.

rescue

(since: 5.9)

Modes allowing mount with damaged filesystem structures.

  • usebackuproot (since: 5.9, replaces standalone option usebackuproot)

  • nologreplay (since: 5.9, replaces standalone option nologreplay)

  • ignorebadroots, ibadroots (since: 5.11)

  • ignoredatacsums, idatacsums (since: 5.11)

  • all (since: 5.9)

skip_balance

(since: 3.3, default: off)

Skip automatic resume of an interrupted balance operation. The operation can later be resumed with btrfs balance resume, or the paused state can be removed with btrfs balance cancel. The default behaviour is to resume an interrupted balance immediately after a volume is mounted.

space_cache, space_cache=<version>, nospace_cache

(nospace_cache since: 3.2, space_cache=v1 and space_cache=v2 since 4.5, default: space_cache=v1)

Options to control the free space cache. The free space cache greatly improves performance when reading block group free space into memory. However, managing the space cache consumes some resources, including a small amount of disk space.

There are two implementations of the free space cache. The original one, referred to as v1, is the safe default. The v1 space cache can be disabled at mount time with nospace_cache without clearing.

On very large filesystems (many terabytes) and certain workloads, the performance of the v1 space cache may degrade drastically. The v2 implementation, which adds a new b-tree called the free space tree, addresses this issue. Once enabled, the v2 space cache will always be used and cannot be disabled unless it is cleared. Use clear_cache,space_cache=v1 or clear_cache,nospace_cache to do so. If v2 is enabled, kernels without v2 support will only be able to mount the filesystem in read-only mode.

The btrfs-check(8) and :doc:`mkfs.btrfs(8)<mkfs.btrfs> commands have full v2 free space cache support since v4.19.

If a version is not explicitly specified, the default implementation will be chosen, which is v1.

ssd, ssd_spread, nossd, nossd_spread

(default: SSD autodetected)

Options to control SSD allocation schemes. By default, BTRFS will enable or disable SSD optimizations depending on status of a device with respect to rotational or non-rotational type. This is determined by the contents of /sys/block/DEV/queue/rotational). If it is 0, the ssd option is turned on. The option nossd will disable the autodetection.

The optimizations make use of the absence of the seek penalty that’s inherent for the rotational devices. The blocks can be typically written faster and are not offloaded to separate threads.

Note

Since 4.14, the block layout optimizations have been dropped. This used to help with first generations of SSD devices. Their FTL (flash translation layer) was not effective and the optimization was supposed to improve the wear by better aligning blocks. This is no longer true with modern SSD devices and the optimization had no real benefit. Furthermore it caused increased fragmentation. The layout tuning has been kept intact for the option ssd_spread.

The ssd_spread mount option attempts to allocate into bigger and aligned chunks of unused space, and may perform better on low-end SSDs. ssd_spread implies ssd, enabling all other SSD heuristics as well. The option nossd will disable all SSD options while nossd_spread only disables ssd_spread.

subvol=<path>

Mount subvolume from path rather than the toplevel subvolume. The path is always treated as relative to the toplevel subvolume. This mount option overrides the default subvolume set for the given filesystem.

subvolid=<subvolid>

Mount subvolume specified by a subvolid number rather than the toplevel subvolume. You can use btrfs subvolume list of btrfs subvolume show to see subvolume ID numbers. This mount option overrides the default subvolume set for the given filesystem.

Note

If both subvolid and subvol are specified, they must point at the same subvolume, otherwise the mount will fail.

thread_pool=<number>

(default: min(NRCPUS + 2, 8) )

The number of worker threads to start. NRCPUS is number of on-line CPUs detected at the time of mount. Small number leads to less parallelism in processing data and metadata, higher numbers could lead to a performance hit due to increased locking contention, process scheduling, cache-line bouncing or costly data transfers between local CPU memories.

treelog, notreelog

(default: on)

Enable the tree logging used for fsync and O_SYNC writes. The tree log stores changes without the need of a full filesystem sync. The log operations are flushed at sync and transaction commit. If the system crashes between two such syncs, the pending tree log operations are replayed during mount.

Warning

Currently, the tree log is replayed even with a read-only mount! To disable that behaviour, also mount with nologreplay.

The tree log could contain new files/directories, these would not exist on a mounted filesystem if the log is not replayed.

usebackuproot

(since: 4.6, default: off)

Enable autorecovery attempts if a bad tree root is found at mount time. Currently this scans a backup list of several previous tree roots and tries to use the first readable. This can be used with read-only mounts as well.

Note

This option has replaced recovery.

user_subvol_rm_allowed

(default: off)

Allow subvolumes to be deleted by their respective owner. Otherwise, only the root user can do that.

Note

Historically, any user could create a snapshot even if he was not owner of the source subvolume, the subvolume deletion has been restricted for that reason. The subvolume creation has been restricted but this mount option is still required. This is a usability issue. Since 4.18, the rmdir(2) syscall can delete an empty subvolume just like an ordinary directory. Whether this is possible can be detected at runtime, see rmdir_subvol feature in FILESYSTEM FEATURES.

DEPRECATED MOUNT OPTIONS

List of mount options that have been removed, kept for backward compatibility.

recovery

(since: 3.2, default: off, deprecated since: 4.5)

Note

This option has been replaced by usebackuproot and should not be used but will work on 4.5+ kernels.

inode_cache, noinode_cache

(removed in: 5.11, since: 3.0, default: off)

Note

The functionality has been removed in 5.11, any stale data created by previous use of the inode_cache option can be removed by btrfs check --clear-ino-cache.

NOTES ON GENERIC MOUNT OPTIONS

Some of the general mount options from mount(8) that affect BTRFS and are worth mentioning.

noatime

under read intensive work-loads, specifying noatime significantly improves performance because no new access time information needs to be written. Without this option, the default is relatime, which only reduces the number of inode atime updates in comparison to the traditional strictatime. The worst case for atime updates under ‘relatime’ occurs when many files are read whose atime is older than 24 h and which are freshly snapshotted. In that case the atime is updated and COW happens - for each file - in bulk. See also https://lwn.net/Articles/499293/ - Atime and btrfs: a bad combination? (LWN, 2012-05-31).

Note that noatime may break applications that rely on atime uptimes like the venerable Mutt (unless you use maildir mailboxes).

Bootloaders

GRUB2 (https://www.gnu.org/software/grub) has the most advanced support of booting from BTRFS with respect to features.

U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for booting but not all BTRFS features are implemented, check the documentation.

EXTLINUX (from the https://syslinux.org project) can boot but does not support all features. Please check the upstream documentation before you use it.

The first 1MiB on each device is unused with the exception of primary superblock that is on the offset 64KiB and spans 4KiB.

Filesystem limits

maximum file name length

255

maximum symlink target length

depends on the nodesize value, for 4KiB it’s 3949 bytes, for larger nodesize it’s 4095 due to the system limit PATH_MAX

The symlink target may not be a valid path, ie. the path name components can exceed the limits (NAME_MAX), there’s no content validation at symlink(3) creation.

maximum number of inodes

264 but depends on the available metadata space as the inodes are created dynamically

inode numbers

minimum number: 256 (for subvolumes), regular files and directories: 257

maximum file length

inherent limit of btrfs is 264 (16 EiB) but the linux VFS limit is 263 (8 EiB)

maximum number of subvolumes

the subvolume ids can go up to 264 but the number of actual subvolumes depends on the available metadata space, the space consumed by all subvolume metadata includes bookkeeping of shared extents can be large (MiB, GiB)

maximum number of hardlinks of a file in a directory

65536 when the extref feature is turned on during mkfs (default), roughly 100 otherwise

minimum filesystem size

the minimal size of each device depends on the mixed-bg feature, without that (the default) it’s about 109MiB, with mixed-bg it’s is 16MiB

Flexibility

The underlying design of BTRFS data structures allows a lot of flexibility and making changes after filesystem creation, like resizing, adding/removing space or enabling some features on-the-fly.

  • dynamic inode creation -- there’s no fixed space or tables for tracking inodes so the number of inodes that can be created is bounded by the metadata space and it’s utilization

  • block group profile change on-the-fly -- the block group profiles can be changed on a mounted filesystem by running the balance operation and specifying the conversion filters

  • resize -- the space occupied by the filesystem on each device can be resized up (grow) or down (shrink) as long as the amount of data can be still contained on the device