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

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. 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
       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)

       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 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.

       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 [(--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 [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
   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
       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
       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
       btrfs is part of btrfs-progs. Please refer to the btrfs wiki
       http://btrfs.wiki.kernel.org for further details.

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

Btrfs v4.20.1                            01/23/2019                          BTRFS-DEVICE(8)

usage: btrfs device <command> [<args>]

    btrfs device add [options] <device> [<device>...] <path>
        Add a device to a filesystem
    btrfs device delete <device>|<devid> [<device>|<devid>...] <path>
    btrfs device remove <device>|<devid> [<device>|<devid>...] <path>
        Remove a device from a filesystem
    btrfs device scan [(-d|--all-devices)|<device> [<device>...]]
        Scan devices for a btrfs filesystem
    btrfs device ready <device>
        Check device to see if it has all of its devices in cache for mounting
    btrfs device stats [options] <path>|<device>
        Show device IO error statistics
    btrfs device usage [options] <path> [<path>..]
        Show detailed information about internal allocations in devices.

manage and query devices in the filesystem