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btrfs-mount - mount options and supported file attributes for the btrfs filesystem
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This document describes mount options specific to the btrfs filesystem.
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Other generic mount options are available,and are described in the
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*alloc_start='bytes'*::
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Debugging option to force all block allocations above a certain
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byte threshold on each block device. The value is specified in
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bytes, optionally with a K, M, or G suffix, case insensitive.
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Disable/enable auto defragmentation.
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Auto defragmentation detects small random writes into files and queue
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them up for the defrag process. Works best for small files;
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Not well suited for large database workloads.
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*check_int*|*check_int_data*|*check_int_print_mask='value'*::
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These debugging options control the behavior of the integrity checking
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module (the BTRFS_FS_CHECK_INTEGRITY config option required). +
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`check_int` enables the integrity checker module, which examines all
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block write requests to ensure on-disk consistency, at a large
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memory and CPU cost. +
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`check_int_data` includes extent data in the integrity checks, and
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implies the check_int option. +
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`check_int_print_mask` takes a bitmask of BTRFSIC_PRINT_MASK_* values
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as defined in 'fs/btrfs/check-integrity.c', to control the integrity
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checker module behavior. +
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See comments at the top of 'fs/btrfs/check-integrity.c'
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Set the interval of periodic commit, 30 seconds by default. Higher
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values defer data being synced to permanent storage with obvious
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consequences when the system crashes. The upper bound is not forced,
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but a warning is printed if it's more than 300 seconds (5 minutes).
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*compress*|*compress='type'*|*compress-force*|*compress-force='type'*::
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Control BTRFS file data compression. Type may be specified as "zlib"
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"lzo" or "no" (for no compression, used for remounting). If no type
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is specified, zlib is used. If compress-force is specified,
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all files will be compressed, whether or not they compress well.
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If compression is enabled, nodatacow and nodatasum are disabled.
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Allow mounts to continue with missing devices. A read-write mount may
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fail with too many devices missing, for example if a stripe member
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is completely missing.
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*device='devicepath'*::
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Specify a device during mount so that ioctls on the control device
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can be avoided. Especially useful when trying to mount a multi-device
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setup as root. May be specified multiple times for multiple devices.
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Disable/enable discard mount option.
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Discard issues frequent commands to let the block device reclaim space
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freed by the filesystem.
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This is useful for SSD devices, thinly provisioned
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LUNs and virtual machine images, but may have a significant
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performance impact. (The fstrim command is also available to
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initiate batch trims from userspace).
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Disable/enable debugging option to be more verbose in some ENOSPC conditions.
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*fatal_errors='action'*::
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Action to take when encountering a fatal error. +
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"bug" - BUG() on a fatal error. This is the default. +
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"panic" - panic() on a fatal error.
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The `flushoncommit` mount option forces any data dirtied by a write in a
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prior transaction to commit as part of the current commit. This makes
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the committed state a fully consistent view of the file system from the
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application's perspective (i.e., it includes all completed file system
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operations). This was previously the behavior only when a snapshot is
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Enable free inode number caching. Defaults to off due to an overflow
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problem when the free space crcs don't fit inside a single page.
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*max_inline='bytes'*::
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Specify the maximum amount of space, in bytes, that can be inlined in
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a metadata B-tree leaf. The value is specified in bytes, optionally
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with a K, M, or G suffix, case insensitive. In practice, this value
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is limited by the root sector size, with some space unavailable due
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to leaf headers. For a 4k sectorsize, max inline data is ~3900 bytes.
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*metadata_ratio='value'*::
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Specify that 1 metadata chunk should be allocated after every
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'value' data chunks. Off by default.
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Enable/disable support for Posix Access Control Lists (ACLs). See the
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`acl`(5) manual page for more information about ACLs.
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ensure that certain IOs make it through the device cache and are on
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persistent storage. If disabled on a device with a volatile
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(non-battery-backed) write-back cache, nobarrier option will lead to
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filesystem corruption on a system crash or power loss.
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Enable/disable data copy-on-write for newly created files.
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Nodatacow implies nodatasum, and disables all compression.
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Enable/disable data checksumming for newly created files.
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Datasum implies datacow.
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Enable/disable the tree logging used for fsync and O_SYNC writes.
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Enable autorecovery attempts if a bad tree root is found at mount time.
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Currently this scans a list of several previous tree roots and tries to
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use the first readable.
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Force check and rebuild procedure of the UUID tree. This should not
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Skip automatic resume of interrupted balance operation after mount.
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May be resumed with "btrfs balance resume."
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Disable freespace cache loading without clearing the cache.
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Force clearing and rebuilding of the disk space cache if something
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*ssd*|*nossd*|*ssd_spread*::
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Options to control ssd allocation schemes. By default, BTRFS will
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enable or disable ssd allocation heuristics depending on whether a
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rotational or nonrotational disk is in use. The ssd and nossd options
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can override this autodetection. +
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The ssd_spread mount option attempts to allocate into big chunks
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of unused space, and may perform better on low-end ssds. ssd_spread
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implies ssd, enabling all other ssd heuristics as well.
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Mount subvolume at 'path' rather than the root subvolume. The
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'path' is relative to the top level subvolume.
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Mount subvolume specified by an ID number rather than the root subvolume.
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This allows mounting of subvolumes which are not in the root of the mounted
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You can use "btrfs subvolume list" to see subvolume ID numbers.
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*subvolrootid='objectid' (deprecated)*::
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Mount subvolume specified by 'objectid' rather than the root subvolume.
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This allows mounting of subvolumes which are not in the root of the mounted
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You can use "btrfs subvolume show" to see the object ID for a subvolume.
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*thread_pool='number'*::
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The number of worker threads to allocate. The default number is equal
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to the number of CPUs + 2, or 8, whichever is smaller.
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*user_subvol_rm_allowed*::
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Allow subvolumes to be deleted by a non-root user. Use with caution.
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The btrfs filesystem supports setting the following file
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attributes the `chattr`(1) utility
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*A* -- no atime updates
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*C* -- no copy on write
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*D* -- synchronous directory updates
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*S* -- synchronous updates
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*X* -- no compression
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For descriptions of these attribute flags, please refer to the
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`chattr`(1) man page.
added
removed
Documentation/btrfs-mount.txt
