bionic (8) btrfs-device.8.gz

Provided by: btrfs-progs_4.15.1-1build1_amd64 bug

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 it this 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 al 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)