BTRFS-DEVICE(8) BTRFS 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 de-
vices. Devices can be then added, removed or replaced on demand. Data
and metadata are organized in allocation profiles with various redun-
dancy 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 fil-
ter convert.
Type The block group profile type is the main distinction of the in-
formation 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 redun-
dancy/replication constraints in connection with the number of
devices. The profile applies to data and metadata block groups
separately. E.g. single, RAID1.
RAID level
Where applicable, the level refers to a profile that matches
constraints of the standard RAID levels. At the moment the sup-
ported ones are: RAID0, RAID1, RAID10, RAID5 and RAID6.
TYPICAL USE CASES
Starting with a single-device filesystem
Assume we've created a filesystem on a block device /dev/sda with pro-
file 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 pro-
files. 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 ex-
actly 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 meta-
data (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 im-
proved redundancy for metadata. If we decide to increase redundancy of
data as well, we're going to lose 50GiB of the second device for obvi-
ous reasons.
$ btrfs balance start -dconvert=raid1 /mnt
The balance process needs some workspace (i.e. 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 de-
vice.
The device size of /dev/sdb as seen by the filesystem remains un-
changed, but the logical space from 50-100GiB will be unused.
Remove device
Device removal must satisfy the profile constraints, otherwise the com-
mand 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
SUBCOMMAND
add [-Kf] <device> [<device>...] <path>
Add device(s) to the filesystem identified by path.
If applicable, a whole device discard (TRIM) operation is per-
formed 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 e.g. 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, oth-
erwise 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 and cannot be enforced. The filesystem must be
converted to profile(s) that would allow the removal. This can
for example happen when going down from 2 devices to 1 and using
the RAID1 profile. See the section Typical use cases.
The operation can take long as it needs to move all data from
the device.
NOTE:
It's possible to specify more than one device on the command
line but the devices will be removed one by one, not at once.
This means that the remaining devices to be deleted can be
still used for writes. In that case there's a warning and
safety timeout as this can be confusing and unexpected. The
timeout can be overridden by option --force.
It is possible to delete the device that was used to mount the
filesystem. The device entry in the mount table
(/proc/self/mounts) 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, oth-
erwise continue
--force
skip the safety timeout when there are multiple devices
for removal, this does not force removing devices that
would break the profile constraints
delete <device>|<devid> [<device>|<devid>...] <path>
Alias of remove kept for backward compatibility
replace <command> [options] <path>
Alias of whole command group btrfs replace for convenience. See
btrfs-replace(8).
ready <device>
Wait until all devices of a multiple-device filesystem are
scanned and registered within the kernel module. This is to pro-
vide a way for automatic filesystem mounting tools to wait be-
fore 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 al-
ready registered remain as such. Reloading the kernel module
will drop this information. There's an alternative way of mount-
ing multiple-device filesystem without the need for prior scan-
ning. 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.
-T Print stats in a tabular form, devices as rows and stats
as columns
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 de-
vice, e.g. 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 de-
fault
-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 prece-
dence.
DEVICE STATS
The device stats keep persistent record of several error classes re-
lated to doing IO. The current values are printed at mount time and up-
dated 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 be-
neath 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 (e.g.
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 re-
turned in case of failure.
If the -c 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 documentation at
https://btrfs.readthedocs.io.
SEE ALSO
btrfs-balance(8) btrfs-device(8), btrfs-replace(8), mkfs.btrfs(8)
6.6.3 Mar 31, 2024 BTRFS-DEVICE(8)
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