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 provided by btrfs device and
The profiles can be also changed, provided there’s enough workspace
to do the conversion, using the btrfs balance command and namely the
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
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] <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.
do not perform discard (TRIM) by default
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
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
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 DEVICESTATS 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 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.
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.
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
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 logical mappings).
· 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.
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
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
The block generation does not match the expected value (eg.
stored in the parent node).
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.
This page is part of the btrfs-progs (btrfs filesystem tools)
project. Information about the project can be found at
If you have a bug report for this manual page, see
This page was obtained from the project's upstream Git repository
on 2017-03-13. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-to-
date source for the page, or you have corrections or improvements to
the information in this COLOPHON (which is not part of the original
manual page), send a mail to email@example.com
Btrfs v4.6.1 03/12/2017 BTRFS-DEVICE(8)