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.
- 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
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, 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 section TYPICAL
USECASES.
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
- 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 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.
- -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 device, 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
- -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
If conflicting options are passed, the last one takes
precedence.
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 (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.
btrfs device returns a zero exit status if it succeeds. Non
zero is returned 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.
btrfs is part of btrfs-progs. Please refer to the
documentation at https://btrfs.readthedocs.io or wiki
http://btrfs.wiki.kernel.org for further information.