btrfs-device(8) — Linux manual page


BTRFS-DEVICE(8)               Btrfs Manual               BTRFS-DEVICE(8)

NAME         top

       btrfs-device - manage devices of btrfs filesystems

SYNOPSIS         top

       btrfs device <subcommand> <args>

DESCRIPTION         top

       The btrfs device command group is used to manage devices of the
       btrfs filesystems.


       Btrfs filesystem can be created on top of single or multiple
       block devices. Data and metadata are organized in allocation
       profiles with various redundancy policies. There’s some
       similarity with traditional RAID levels, but this could be
       confusing to users familiar with the traditional meaning. Due to
       the similarity, the RAID terminology is widely used in the
       documentation. See mkfs.btrfs(8) for more details and the exact
       profile capabilities and constraints.

       The device management works on a mounted filesystem. Devices can
       be added, removed or replaced, by commands provided by btrfs
       device and btrfs replace.

       The profiles can be also changed, provided there’s enough
       workspace to do the conversion, using the btrfs balance command
       and namely the filter convert.

           The block group profile type is the main distinction of the
           information stored on the block device. User data are called
           Data, the internal data structures managed by filesystem are
           Metadata and System.

           A profile describes an allocation policy based on the
           redundancy/replication constraints in connection with the
           number of devices. The profile applies to data and metadata
           block groups separately. Eg.  single, RAID1.

       RAID level
           Where applicable, the level refers to a profile that matches
           constraints of the standard RAID levels. At the moment the
           supported ones are: RAID0, RAID1, RAID10, RAID5 and RAID6.

       See the section TYPICAL USECASES for some examples.

SUBCOMMAND         top

       add [-Kf] <device> [<device>...] <path>
           Add device(s) to the filesystem identified by <path>.

           If applicable, a whole device discard (TRIM) operation is
           performed prior to adding the device. A device with existing
           filesystem detected by blkid(8) will prevent device addition
           and has to be forced. Alternatively the filesystem can be
           wiped from the device using eg. the wipefs(8) tool.

           The operation is instant and does not affect existing data.
           The operation merely adds the device to the filesystem
           structures and creates some block groups headers.


               do not perform discard (TRIM) by default

               force overwrite of existing filesystem on the given

               wait if there’s another exclusive operation running,
               otherwise continue

       remove [options] <device>|<devid> [<device>|<devid>...] <path>
           Remove device(s) from a filesystem identified by <path>

           Device removal must satisfy the profile constraints,
           otherwise the command fails. The filesystem must be converted
           to profile(s) that would allow the removal. This can
           typically happen when going down from 2 devices to 1 and
           using the RAID1 profile. See the TYPICAL USECASES section

           The operation can take long as it needs to move all data from
           the device.

           It is possible to delete the device that was used to mount
           the filesystem. The device entry in the mount table will be
           replaced by another device name with the lowest device id.

           If the filesystem is mounted in degraded mode (-o degraded),
           special term missing can be used for device. In that case,
           the first device that is described by the filesystem
           metadata, but not present at the mount time will be removed.

               In most cases, there is only one missing device in
               degraded mode, otherwise mount fails. If there are two or
               more devices missing (e.g. possible in RAID6), you need
               specify missing as many times as the number of missing
               devices to remove all of them.

               wait if there’s another exclusive operation running,
               otherwise continue

       delete <device>|<devid> [<device>|<devid>...] <path>
           Alias of remove kept for backward compatibility

       ready <device>
           Wait until all devices of a multiple-device filesystem are
           scanned and registered within the kernel module. This is to
           provide a way for automatic filesystem mounting tools to wait
           before the mount can start. The device scan is only one of
           the preconditions and the mount can fail for other reasons.
           Normal users usually do not need this command and may safely
           ignore it.

       scan [options] [<device> [<device>...]]
           Scan devices for a btrfs filesystem and register them with
           the kernel module. This allows mounting multiple-device
           filesystem by specifying just one from the whole group.

           If no devices are passed, all block devices that blkid
           reports to contain btrfs are scanned.

           The options --all-devices or -d can be used as a fallback in
           case blkid is not available. If used, behavior is the same as
           if no devices are passed.

           The command can be run repeatedly. Devices that have been
           already registered remain as such. Reloading the kernel
           module will drop this information. There’s an alternative way
           of mounting multiple-device filesystem without the need for
           prior scanning. See the mount option device.


               Enumerate and register all devices, use as a fallback in
               case blkid is not available.

               Unregister a given device or all stale devices if no path
               is given, the device must be unmounted otherwise it’s an

       stats [options] <path>|<device>
           Read and print the device IO error statistics for all devices
           of the given filesystem identified by <path> or for a single
           <device>. The filesystem must be mounted. See section DEVICE
           STATS for more information about the reported statistics and
           the meaning.


               Print the stats and reset the values to zero afterwards.

               Check if the stats are all zeros and return 0 if it is
               so. Set bit 6 of the return code if any of the statistics
               is no-zero. The error values is 65 if reading stats from
               at least one device failed, otherwise it’s 64.

       usage [options] <path> [<path>...]
           Show detailed information about internal allocations on

           The level of detail can differ if the command is run under a
           regular or the root user (due to use of restricted ioctls).
           The first example below is for normal user (warning included)
           and the next one with root on the same filesystem:

               WARNING: cannot read detailed chunk info, per-device usage will not be shown, run as root
               /dev/sdc1, ID: 1
                  Device size:           931.51GiB
                  Device slack:              0.00B
                  Unallocated:           931.51GiB

               /dev/sdc1, ID: 1
                  Device size:           931.51GiB
                  Device slack:              0.00B
                  Data,single:           641.00GiB
                  Data,RAID0/3:            1.00GiB
                  Metadata,single:        19.00GiB
                  System,single:          32.00MiB
                  Unallocated:           271.48GiB

           •   Device size — size of the device as seen by the
               filesystem (may be different than actual device size)

           •   Device slack — portion of device not used by the
               filesystem but still available in the physical space
               provided by the device, eg. after a device shrink

           •   Data,single, Metadata,single, System,single — in general,
               list of block group type (Data, Metadata, System) and
               profile (single, RAID1, ...) allocated on the device

           •   Data,RAID0/3 — in particular, striped profiles
               RAID0/RAID10/RAID5/RAID6 with the number of devices on
               which the stripes are allocated, multiple occurrences of
               the same profile can appear in case a new device has been
               added and all new available stripes have been used for

           •   Unallocated — remaining space that the filesystem can
               still use for new block groups


               raw numbers in bytes, without the B suffix

               print human friendly numbers, base 1024, this is the

               print human friendly numbers, base 1000

               select the 1024 base for the following options, according
               to the IEC standard

               select the 1000 base for the following options, according
               to the SI standard

               show sizes in KiB, or kB with --si

               show sizes in MiB, or MB with --si

               show sizes in GiB, or GB with --si

               show sizes in TiB, or TB with --si

       If conflicting options are passed, the last one takes precedence.


       Assume we’ve created a filesystem on a block device /dev/sda with
       profile single/single (data/metadata), the device size is 50GiB
       and we’ve used the whole device for the filesystem. The mount
       point is /mnt.

       The amount of data stored is 16GiB, metadata have allocated 2GiB.


           We want to increase the total size of the filesystem and keep
           the profiles. The size of the new device /dev/sdb is 100GiB.

               $ btrfs device add /dev/sdb /mnt

           The amount of free data space increases by less than 100GiB,
           some space is allocated for metadata.


           Now we want to increase the redundancy level of both data and
           metadata, but we’ll do that in steps. Note, that the device
           sizes are not equal and we’ll use that to show the
           capabilities of split data/metadata and independent profiles.

           The constraint for RAID1 gives us at most 50GiB of usable
           space and exactly 2 copies will be stored on the devices.

           First we’ll convert the metadata. As the metadata occupy less
           than 50GiB and there’s enough workspace for the conversion
           process, we can do:

               $ btrfs balance start -mconvert=raid1 /mnt

           This operation can take a while, because all metadata have to
           be moved and all block pointers updated. Depending on the
           physical locations of the old and new blocks, the disk
           seeking is the key factor affecting performance.

           You’ll note that the system block group has been also
           converted to RAID1, this normally happens as the system block
           group also holds metadata (the physical to logical mappings).

           What changed:

           •   available data space decreased by 3GiB, usable roughly
               (50 - 3) + (100 - 3) = 144 GiB

           •   metadata redundancy increased

           IOW, the unequal device sizes allow for combined space for
           data yet improved redundancy for metadata. If we decide to
           increase redundancy of data as well, we’re going to lose
           50GiB of the second device for obvious reasons.

               $ btrfs balance start -dconvert=raid1 /mnt

           The balance process needs some workspace (ie. a free device
           space without any data or metadata block groups) so the
           command could fail if there’s too much data or the block
           groups occupy the whole first device.

           The device size of /dev/sdb as seen by the filesystem remains
           unchanged, but the logical space from 50-100GiB will be


           Device removal must satisfy the profile constraints,
           otherwise the command fails. For example:

               $ btrfs device remove /dev/sda /mnt
               ERROR: error removing device '/dev/sda': unable to go below two devices on raid1

           In order to remove a device, you need to convert the profile
           in this case:

               $ btrfs balance start -mconvert=dup -dconvert=single /mnt
               $ btrfs device remove /dev/sda /mnt

DEVICE STATS         top

       The device stats keep persistent record of several error classes
       related to doing IO. The current values are printed at mount time
       and updated during filesystem lifetime or from a scrub run.

           $ btrfs device stats /dev/sda3
           [/dev/sda3].write_io_errs   0
           [/dev/sda3].read_io_errs    0
           [/dev/sda3].flush_io_errs   0
           [/dev/sda3].corruption_errs 0
           [/dev/sda3].generation_errs 0

           Failed writes to the block devices, means that the layers
           beneath the filesystem were not able to satisfy the write

           Read request analogy to write_io_errs.

           Number of failed writes with the FLUSH flag set. The flushing
           is a method of forcing a particular order between write
           requests and is crucial for implementing crash consistency.
           In case of btrfs, all the metadata blocks must be permanently
           stored on the block device before the superblock is written.

           A block checksum mismatched or a corrupted metadata header
           was found.

           The block generation does not match the expected value (eg.
           stored in the parent node).

       Since kernel 5.14 the device stats are also available in textual
       form in /sys/fs/btrfs/FSID/devinfo/DEVID/error_stats.

EXIT STATUS         top

       btrfs device returns a zero exit status if it succeeds. Non zero
       is returned in case of failure.

       If the -s option is used, btrfs device stats will add 64 to the
       exit status if any of the error counters is non-zero.

AVAILABILITY         top

       btrfs is part of btrfs-progs. Please refer to the btrfs wiki for further details.

SEE ALSO         top

       mkfs.btrfs(8), btrfs-replace(8), btrfs-balance(8)

COLOPHON         top

       This page is part of the btrfs-progs (btrfs filesystem tools)
       project.  Information about the project can be found at 
       If you have a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       on 2023-12-22.  (At that time, the date of the most recent commit
       that was found in the repository was 2023-12-14.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to

Btrfs v5.16.1                  02/06/2022                BTRFS-DEVICE(8)

Pages that refer to this page: btrfs(8)btrfs-balance(8)btrfs-replace(8)