Provided by: btrfs-progs_5.16.2-1_amd64 
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.
Type
The block group profile type is the main distinction of the information stored on the
block device. User data are called Data, the internal data structures managed by
filesystem are Metadata and System.
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. Eg. single, RAID1.
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)
--enqueue
wait if there’s another exclusive operation running, otherwise continue
remove [options] <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.
Options
--enqueue
wait if there’s another exclusive operation running, otherwise continue
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
group.
If no devices are passed, all block devices that blkid reports to contain btrfs are
scanned.
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.
Options
-d|--all-devices
Enumerate and register all devices, use as a fallback in case blkid is not
available.
-u|--forget
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
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 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 on devices.
The level of detail can differ if the command is run under a regular or the root user
(due to use of restricted ioctls). The first example below is for normal user (warning
included) and the next one with root on the same filesystem:
WARNING: cannot read detailed chunk info, per-device usage will not be shown, run as root
/dev/sdc1, ID: 1
Device size: 931.51GiB
Device slack: 0.00B
Unallocated: 931.51GiB
/dev/sdc1, ID: 1
Device size: 931.51GiB
Device slack: 0.00B
Data,single: 641.00GiB
Data,RAID0/3: 1.00GiB
Metadata,single: 19.00GiB
System,single: 32.00MiB
Unallocated: 271.48GiB
• Device size — size of the device as seen by the filesystem (may be different than
actual device size)
• Device slack — portion of device not used by the filesystem but still available in
the physical space provided by the device, eg. after a device shrink
• Data,single, Metadata,single, System,single — in general, list of block group type
(Data, Metadata, System) and profile (single, RAID1, ...) allocated on the device
• Data,RAID0/3 — in particular, striped profiles RAID0/RAID10/RAID5/RAID6 with the
number of devices on which the stripes are allocated, multiple occurrences of the
same profile can appear in case a new device has been added and all new available
stripes have been used for writes
• Unallocated — remaining space that the filesystem can still use for new block
groups
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 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
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).
Since kernel 5.14 the device stats are also available in textual form in
/sys/fs/btrfs/FSID/devinfo/DEVID/error_stats.
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)