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BTRFS-DEVICE(8) Btrfs Manual BTRFS-DEVICE(8)

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 [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 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 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:

o available data space decreased by 3GiB, usable roughly (50 - 3)
+ (100 - 3) = 144 GiB

o 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)

Btrfs v5.4.1 01/09/2020 BTRFS-DEVICE(8)