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.
See the section TYPICAL USECASES for some examples.
add [-Kf] <device> [<device>...] <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.
remove [options] <device>|<devid> [<device>|<devid>...] <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.
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>
scan [options] [<device> [<device>...]]
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.
stats [options] <path>|<device>
usage [options] <path> [<path>...]
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).
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.
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
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.