Provided by: btrfs-progs_6.6.3-1.1build2_amd64 bug

NAME

       btrfs-device - manage devices of btrfs filesystems

SYNOPSIS

       btrfs device <subcommand> <args>

DESCRIPTION

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

DEVICE MANAGEMENT

       BTRFS  filesystem  can be created on top of single or multiple block devices.  Devices can
       be then added, removed or  replaced  on  demand.   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.

       Type   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.

       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. E.g. 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.

TYPICAL USE CASES

   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 (i.e. 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

SUBCOMMAND

       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 e.g. 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
              and  cannot  be enforced. The filesystem must be converted to profile(s) that would
              allow the removal. This can for example happen when going down from 2 devices to  1
              and using the RAID1 profile. See the section Typical use cases.

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

              NOTE:
                 It's  possible  to  specify  more  than  one  device on the command line but the
                 devices will be removed one by one, not at once.  This means that the  remaining
                 devices  to  be  deleted  can  be  still used for writes. In that case there's a
                 warning and safety timeout as this can be confusing and unexpected. The  timeout
                 can be overridden by option --force.

              It  is  possible  to  delete  the device that was used to mount the filesystem. The
              device entry in the mount table (/proc/self/mounts) 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

              --force
                     skip the safety timeout when there are multiple devices  for  removal,  this
                     does not force removing devices that would break the profile constraints

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

       replace <command> [options] <path>
              Alias of whole command group btrfs replace for convenience. See btrfs-replace(8).

       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.

              -T     Print stats in a tabular form, devices as rows and stats as columns

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

              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, e.g.  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 writes

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

              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.

DEVICE STATS

       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 (e.g. 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

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

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

AVAILABILITY

       btrfs   is   part   of   btrfs-progs.    Please   refer   to    the    documentation    at
       https://btrfs.readthedocs.io.

SEE ALSO

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