btrfs-progs/Documentation/btrfs-device.rst

btrfs-device(8)
===============

SYNOPSIS
--------

**btrfs device** <subcommand> <args>

DESCRIPTION
-----------

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

DEVICE MANAGEMENT
-----------------

.. include ch-volume-management-intro.rst

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)

        --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 section *TYPICAL USECASES*.

        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

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:

        .. code-block:: none

                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, eg.
          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.

.. code-block:: none

        $ 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).

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 btrfs wiki http://btrfs.wiki.kernel.org for
further details.

SEE ALSO
--------

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