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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(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 command and namely the
       filter convert.
       Profile
           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
           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] <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.
           Options
           -K|--nodiscard
               do not perform discard (TRIM) by default
           -f|--force
               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
           device.
               Note
               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 [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 it 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 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:
           o   available data space decreased by 3GiB, usable roughly (50 - 3)
               + (100 - 3) = 144 GiB
           o   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.
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 v4.9.1                      08/06/2017                   BTRFS-DEVICE(8)