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NAME | SYNOPSIS | DESCRIPTION | DEVICE MANAGEMENT | SUBCOMMAND | TYPICAL USECASES | DEVICE STATS | EXIT STATUS | AVAILABILITY | SEE ALSO | COLOPHON |
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BTRFS-DEVICE(8) Btrfs Manual BTRFS-DEVICE(8)
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.
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.
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.
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
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.
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.
btrfs is part of btrfs-progs. Please refer to the btrfs wiki
http://btrfs.wiki.kernel.org for further details.
mkfs.btrfs(8), btrfs-replace(8), btrfs-balance(8)
This page is part of the btrfs-progs (btrfs filesystem tools)
project. Information about the project can be found at
⟨https://btrfs.wiki.kernel.org/index.php/Btrfs_source_repositories⟩.
If you have a bug report for this manual page, see
⟨https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#How_do_I_report_bugs_and_issues.3F⟩.
This page was obtained from the project's upstream Git repository
⟨git://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git⟩
on 2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-02.) 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 man-pages@man7.org
Btrfs v5.16.1 02/06/2022 BTRFS-DEVICE(8)
Pages that refer to this page: btrfs(8), btrfs-balance(8), btrfs-replace(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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