Provided by: btrfs-progs_5.2.1-1ubuntu1_amd64 bug


       btrfs-device - manage devices of btrfs filesystems


       btrfs device <subcommand> <args>


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


       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.

           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.


       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.


               do not perform discard (TRIM) by default

               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.

               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

       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 [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

           If no devices are passed, all block devices that blkid reports to contain btrfs are

           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.


               Enumerate and register all devices, use as a fallback in case blkid is not

               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


               Print the stats and reset the values to zero afterwards.

               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.


               raw numbers in bytes, without the B suffix

               print human friendly numbers, base 1024, this is the default

               print human friendly numbers, base 1000

               select the 1024 base for the following options, according to the IEC standard

               select the 1000 base for the following options, according to the SI standard

               show sizes in KiB, or kB with --si

               show sizes in MiB, or MB with --si

               show sizes in GiB, or GB with --si

               show sizes in TiB, or TB with --si

       If conflicting options are passed, the last one takes precedence.


       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.

           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.

           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

           ·   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 removal must satisfy the profile constraints, otherwise the command fails. For

               $ 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


       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

           Failed writes to the block devices, means that the layers beneath the filesystem were
           not able to satisfy the write request.

           Read request analogy to write_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.

           A block checksum mismatched or a corrupted metadata header was found.

           The block generation does not match the expected value (eg. stored in the parent


       btrfs device returns a zero exit status if it succeeds. Non zero is returned in case of

       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.


       btrfs is part of btrfs-progs. Please refer to the btrfs wiki
       for further details.


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