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


       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.

           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

       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 disk(s)

       remove <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 below.

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

           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

       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 error.

       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


               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 in devices.


               raw numbers in bytes, without the B suffix

               print human friendly numbers, base 1024, this is the default

               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

       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 unused.

           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 request.

           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

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

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 2020-09-18.  (At that time, the date of the most recent commit
       that was found in the repository was 2020-07-02.)  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

Btrfs v4.6.1                     05/16/2020                  BTRFS-DEVICE(8)

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