btrfs-device(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | DEVICE MANAGEMENT | SUBCOMMAND | TYPICAL USECASES | DEVICE STATS | EXIT STATUS | AVAILABILITY | SEE ALSO | COLOPHON

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.

DEVICE MANAGEMENT         top

       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.

       Profile
           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.

       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.

           Options

           -K|--nodiscard
               do not perform discard (TRIM) by default

           -f|--force
               force overwrite of existing filesystem on the given
               disk(s)

           --enqueue
               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
           below.

           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.

               Note
               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.
           Options

           --enqueue
               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.

           Options

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

           -u|--forget
               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 meaning.

           Options

           -z|--reset
               Print the stats and reset the values to zero afterwards.

           -c|--check
               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.

           Options

           -b|--raw
               raw numbers in bytes, without the B suffix

           -h|--human-readable
               print human friendly numbers, base 1024, this is the
               default

           -H
               print human friendly numbers, base 1000

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

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

           -k|--kbytes
               show sizes in KiB, or kB with --si

           -m|--mbytes
               show sizes in MiB, or MB with --si

           -g|--gbytes
               show sizes in GiB, or GB with --si

           -t|--tbytes
               show sizes in TiB, or TB with --si

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

TYPICAL USECASES         top

   STARTING WITH A SINGLE-DEVICE FILESYSTEM
       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.

       ADD NEW DEVICE
           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.

       CONVERT TO RAID1
           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.

       REMOVE DEVICE
           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

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

       read_io_errors
           Read request analogy to write_io_errs.

       flush_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.

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

       generation_errs
           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
       http://btrfs.wiki.kernel.org 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 
       ⟨https://btrfs.wiki.kernel.org/index.php/Btrfs_source_repositories⟩.
       If you have a bug report for this manual page, see
       ⟨https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#How_do_I_report_bugs_and_issues.3F⟩.
       This page was obtained from the project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git⟩
       on 2021-04-01.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-24.)  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 man-pages@man7.org

Btrfs v4.6.1                   03/13/2021                BTRFS-DEVICE(8)

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