btrfs-device - manage devices of btrfs filesystems


   btrfs device <subcommand> <args>


   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(9) 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 profided 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 comand and namely the filter

       A profile describes an allocation policy based on the
       redundancy/replication constrants 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.


   add [-Kf] <dev> [<dev>...] <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 <dev>|<devid> [<dev>|<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 example
       section below.

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

           It is not possible to delete the device that was used to mount
           the filesystem. This is a limitation given by the VFS.

   delete <dev>|<devid> [<dev>|<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.

   scan [(--all-devices|-d)|<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 are deprecated and kept for
       backward compatibility. 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.

   stats [-z] <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>.
       See section DEVICE STATS for more information.


           Print the stats and reset the values to zero afterwards.

   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 as the 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 logial mappings).

       What changed:

       *   available data space decreased by 3GiB, usable rougly (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.


   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).


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


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


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


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