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BTRFS-DEVICE(8) | Btrfs Manual | BTRFS-DEVICE(8) |
NAME¶
btrfs-device - manage devices of btrfs filesystemsSYNOPSIS¶
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(9) 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 profided 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 comand and namely the filter convert. ProfileA profile describes an allocation policy based on the
redundancy/replication constrants 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] <dev> [<dev>...] <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
remove <dev>|<devid>
[<dev>|<devid>...] <path>
do not perform discard (TRIM) by default
-f|--force
force overwrite of existing filesystem on the given
disk(s)
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 example section below.
The operation can take long as it needs to move all data from the device.
Note
It is not possible to delete the device that was used to mount the filesystem.
This is a limitation given by the VFS.
delete <dev>|<devid>
[<dev>|<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.
scan [(--all-devices|-d)|<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 are deprecated and kept for
backward compatibility. 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.
stats [-z] <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>. See section DEVICE STATS for more
information.
Options
-z
usage [options] <path> [<path>...]
Print the stats and reset the values to zero
afterwards.
Show detailed information about internal allocations in
devices.
Options
-b|--raw
If conflicting options are passed, the last one takes precedence.
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
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.
We want to increase the total size of the filesystem and keep the profiles. The
size of the new device /dev/sdb is 100GiB.
The amount of free data space increases by less than 100GiB, some space is
allocated for metadata.
$ btrfs device add /dev/sdb /mnt
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:
This operation can take a while as the 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 logial mappings).
What changed:
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.
$ btrfs balance start -mconvert=raid1 /mnt
•available data space decreased by 3GiB, usable
rougly (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
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
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.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)10/02/2016 | Btrfs v4.7.3 |