BTRFS-MOUNT(5)                             Btrfs Manual                            BTRFS-MOUNT(5)



NAME
       btrfs-mount - mount options and supported file attributes for the btrfs filesystem

DESCRIPTION
       This document describes mount options specific to the btrfs filesystem. Other generic
       mount options are available,and are described in the mount(8) manpage.

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.

       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.

       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).

       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.

       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.

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

       nobarrier
           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.

       nodatacow
           Enable/disable data copy-on-write for newly created files. Nodatacow implies
           nodatasum, and disables all compression.

       nodatasum
           Enable/disable data checksumming for newly created files. Datasum implies datacow.

       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."

       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. The 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.

FILE ATTRIBUTES
       The btrfs filesystem supports setting the following file attributes the chattr(1) utility

       a — append only

       A — no atime updates

       c — compressed

       C — no copy on write

       d — no dump

       D — synchronous directory updates

       i — immutable

       S — synchronous updates

       X — no compression

       For descriptions of these attribute flags, please refer to the chattr(1) man page.

SEE ALSO
       chattr(1), mkfs.btrfs(8), mount(8), btrfs(8)



Btrfs v3.17                                 11/22/2014                             BTRFS-MOUNT(5)