245 lines
8.5 KiB
Plaintext
245 lines
8.5 KiB
Plaintext
btrfs-device(8)
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===============
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NAME
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----
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btrfs-device - manage devices of btrfs filesystems
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SYNOPSIS
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--------
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*btrfs device* <subcommand> <args>
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DESCRIPTION
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-----------
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The *btrfs device* command group is used to manage devices of the btrfs filesystems.
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DEVICE MANAGEMENT
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-----------------
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Btrfs filesystem can be created on top of single or multiple block devices.
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Data and metadata are organized in allocation profiles with various redundancy
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policies. There's some similarity with traditional RAID levels, but this could
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be confusing to users familiar with the traditional meaning. Due to the
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similarity, the RAID terminology is widely used in the documentation. See
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`mkfs.btrfs`(9) for more details and the exact profile capabilities and
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constraints.
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The device management works on a mounted filesystem. Devices can be added,
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removed or replaced, by commands profided by *btrfs device* and *btrfs replace*.
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The profiles can be also changed, provided there's enough workspace to do the
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conversion, using the *btrfs balance* command and namely the filter 'convert'.
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Profile::
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A profile describes an allocation policy based on the redundancy/replication
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constrants in connection with the number of devices. The profile applies to
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data and metadata block groups separately.
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RAID level::
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Where applicable, the level refers to a profile that matches constraints of the
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standard RAID levels. At the moment the supported ones are: RAID0, RAID1,
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RAID10, RAID5 and RAID6.
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See the section *TYPICAL USECASES* for some examples.
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SUBCOMMAND
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----------
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*add* [-Kf] <dev> [<dev>...] <path>::
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Add device(s) to the filesystem identified by <path>.
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If applicable, a whole device discard (TRIM) operation is performed prior to
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adding the device. A device with existing filesystem detected by `blkid`(8)
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will prevent device addition and has to be forced. Alternatively the filesystem
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can be wiped from the device using eg. the `wipefs`(8) tool.
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The operation is instant and does not affect existing data. The operation merely
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adds the device to the filesystem structures and creates some block groups
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headers.
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`Options`
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-K|--nodiscard::::
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do not perform discard (TRIM) by default
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-f|--force::::
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force overwrite of existing filesystem on the given disk(s)
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*remove* <dev>|<devid> [<dev>|<devid>...] <path>::
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Remove device(s) from a filesystem identified by <path>
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Device removal must satisfy the profile constraints, otherwise the command
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fails. The filesystem must be converted to profile(s) that would allow the
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removal. This can typically happen when going down from 2 devices to 1 and
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using the RAID1 profile. See the example section below.
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The operation can take long as it needs to move all data from the device.
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NOTE: It is not possible to delete the device that was used to mount the
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filesystem. This is a limitation given by the VFS.
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*delete* <dev>|<devid> [<dev>|<devid>...] <path>::
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Alias of remove kept for backward compatibility
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*ready* <device>::
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Wait until all devices of a multiple-device filesystem are scanned and registered
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within the kernel module.
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*scan* [(--all-devices|-d)|<device> [<device>...]]::
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Scan devices for a btrfs filesystem and register them with the kernel module.
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This allows mounting multiple-device filesystem by specifying just one from the
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whole group.
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If no devices are passed, all block devices that blkid reports to contain btrfs
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are scanned.
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The options '--all-devices' or '-d' are deprecated and kept for backward compatibility.
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If used, behavior is the same as if no devices are passed.
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The command can be run repeatedly. Devices that have been already registered
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remain as such. Reloading the kernel module will drop this information. There's
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an alternative way of mounting multiple-device filesystem without the need for
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prior scanning. See the mount option 'device'.
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*stats* [-z] <path>|<device>::
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Read and print the device IO error statistics for all devices of the given
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filesystem identified by <path> or for a single <device>. See section *DEVICE
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STATS* for more information.
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`Options`
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-z::::
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Print the stats and reset the values to zero afterwards.
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*usage* [options] <path> [<path>...]::
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Show detailed information about internal allocations in devices.
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`Options`
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-b|--raw::::
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raw numbers in bytes, without the 'B' suffix
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-h|--human-readable::::
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print human friendly numbers, base 1024, this is the default
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-H::::
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print human friendly numbers, base 1000
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--iec::::
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select the 1024 base for the following options, according to the IEC standard
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--si::::
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select the 1000 base for the following options, according to the SI standard
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-k|--kbytes::::
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show sizes in KiB, or kB with --si
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-m|--mbytes::::
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show sizes in MiB, or MB with --si
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-g|--gbytes::::
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show sizes in GiB, or GB with --si
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-t|--tbytes::::
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show sizes in TiB, or TB with --si
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If conflicting options are passed, the last one takes precedence.
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TYPICAL USECASES
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----------------
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STARTING WITH A SINGLE-DEVICE FILESYSTEM
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Assume we've created a filesystem on a block device '/dev/sda' with profile
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'single/single' (data/metadata), the device size is 50GiB and we've used the
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whole device for the filesystem. The mount point is '/mnt'.
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The amount of data stored is 16GiB, metadata have allocated 2GiB.
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==== ADD NEW DEVICE ====
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We want to increase the total size of the filesystem and keep the profiles. The
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size of the new device '/dev/sdb' is 100GiB.
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$ btrfs device add /dev/sdb /mnt
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The amount of free data space increases by less than 100GiB, some space is
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allocated for metadata.
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==== CONVERT TO RAID1 ====
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Now we want to increase the redundancy level of both data and metadata, but
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we'll do that in steps. Note, that the device sizes are not equal and we'll use
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that to show the capabilities of split data/metadata and independent profiles.
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The constraint for RAID1 gives us at most 50GiB of usable space and exactly 2
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copies will be stored on the devices.
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First we'll convert the metadata. As the metadata occupy less than 50GiB and
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there's enough workspace for the conversion process, we can do:
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$ btrfs balance start -mconvert=raid1 /mnt
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This operation can take a while as the metadata have to be moved and all block
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pointers updated. Depending on the physical locations of the old and new
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blocks, the disk seeking is the key factor affecting performance.
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You'll note that the system block group has been also converted to RAID1, this
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normally happens as the system block group also holds metadata (the physical to
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logial mappings).
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What changed:
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* available data space decreased by 3GiB, usable rougly (50 - 3) + (100 - 3) = 144 GiB
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* metadata redundancy increased
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IOW, the unequal device sizes allow for combined space for data yet improved
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redundancy for metadata. If we decide to increase redundancy of data as well,
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we're going to lose 50GiB of the second device for obvious reasons.
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$ btrfs balance start -dconvert=raid1 /mnt
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The balance process needs some workspace (ie. a free device space without any
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data or metadata block groups) so the command could fail if there's too much
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data or the block groups occupy the whole first device.
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The device size of '/dev/sdb' as seen by the filesystem remains unchanged, but
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the logical space from 50-100GiB will be unused.
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DEVICE STATS
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------------
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The device stats keep persistent record of several error classes related to
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doing IO. The current values are printed at mount time and updated during
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filesystem lifetime or from a scrub run.
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$ btrfs device stats /dev/sda3
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[/dev/sda3].write_io_errs 0
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[/dev/sda3].read_io_errs 0
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[/dev/sda3].flush_io_errs 0
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[/dev/sda3].corruption_errs 0
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[/dev/sda3].generation_errs 0
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write_io_errs::
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Failed writes to the block devices, means that the layers beneath the
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filesystem were not able to satisfy the write request.
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read_io_errors::
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Read request analogy to write_io_errs.
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flush_io_errs::
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Number of failed writes with the 'FLUSH' flag set. The flushing is a method of
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forcing a particular order between write requests and is crucial for
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implementing crash consistency. In case of btrfs, all the metadata blocks must
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be permanently stored on the block device before the superblock is written.
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corruption_errs::
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A block checksum mismatched or a corrupted metadata header was found.
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generation_errs::
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The block generation does not match the expected value (eg. stored in the
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parent node).
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EXIT STATUS
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-----------
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*btrfs device* returns a zero exit status if it succeeds. Non zero is
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returned in case of failure.
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AVAILABILITY
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------------
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*btrfs* is part of btrfs-progs.
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Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for
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further details.
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SEE ALSO
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--------
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`mkfs.btrfs`(8),
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`btrfs-replace`(8),
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`btrfs-balance`(8)
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