btrfs-balance man page
btrfs-balance — balance block groups on a btrfs filesystem
btrfs balance <subcommand> <args>
The primary purpose of the balance feature is to spread block groups accross all devices so they match constraints defined by the respective profiles. See
mkfs.btrfs(8) section PROFILES for more details. The scope of the balancing process can be further tuned by use of filters that can select the block groups to process. Balance works only on a mounted filesystem.
The balance operation is cancellable by the user. The on-disk state of the filesystem is always consistent so an unexpected interruption (eg. system crash, reboot) does not corrupt the filesystem. The progress of the balance operation is temporarily stored and will be resumed upon mount, unless the mount option skip_balance is specified.
running balance without filters will take a lot of time as it basically rewrites the entire filesystem and needs to update all block pointers.
The filters can be used to perform following actions:
- convert block group profiles (filter convert)
- make block group usage more compact (filter usage)
- perform actions only on a given device (filters devid, drange)
The filters can be applied to a combination of block group types (data, metadata, system). Note that changing system needs the force option.
The balance subcommand also exists under the btrfs filesystem namespace. This still works for backward compatibility but is deprecated and should not be used anymore.
A short syntax btrfs balance <path> works due to backward compatibility but is deprecated and should not be used anymore. Use btrfs balance start command instead.
- cancel <path>
- cancel running or paused balance
- pause <path>
- pause running balance operation, this will store the state of the balance progress and used filters to the filesystem
- resume <path>
- resume interrupted balance
- start [options] <path>
- start the balance operation according to the specified filters, no filters will rewrite the entire filesystem. The process runs in the foreground.
- act on data block groups, see
Filterssection for details about filters
- act on metadata chunks, see
Filterssection for details about filters
- act on system chunks (requires -f), see
Filterssection for details about filters.
- be verbose and print balance filter arguments
- force reducing of metadata integrity, eg. when going from raid1 to single
- status [-v] <path>
- Show status of running or paused balance.
If -v option is given, output will be verbose.
From kernel 3.3 onwards, btrfs balance can limit its action to a subset of the full filesystem, and can be used to change the replication configuration (e.g. moving data from single to RAID1). This functionality is accessed through the -d, -m or -s options to btrfs balance start, which filter on data, metadata and system blocks respectively.
A filter has the following stucture: type[=params][,type=…]
The available types are:
- Balances only block groups with the given profiles. Parameters are a list of profile names separated by "|" (pipe).
- usage=<percent> , usage=<range>
- Balances only block groups with usage under the given percentage. The value of 0 is allowed and will clean up completely unused block groups, this should not require any new work space allocated. You may want to use usage=0 in case balance is returnin Enospc and your filesystem is not too full.
The argument may be a single value or a range. The single value N means at most N percent used, equivalent to ..N range syntax. Kernels prior to 4.4 accept only the single value format. The minimum range boundary is inclusive, maximum is exclusive.
- Balances only block groups which have at least one chunk on the given device. To list devices with ids use btrfs fi show.
- Balance only block groups which overlap with the given byte range on any device. Use in conjunction with devid to filter on a specific device. The parameter is a range specified as start..end.
- Balance only block groups which overlap with the given byte range in the filesystem’s internal virtual address space. This is the address space that most reports from btrfs in the kernel log use. The parameter is a range specified as start..end.
- Convert each selected block group to the given profile name identified by parameters.
starting with kernel 4.5, the data chunks can be converted to/from the DUP profile on a single device.
- limit=<number> , limit=<range>
- Process only given number of chunks, after all filters are applied. This can be used to specifically target a chunk in connection with other filters (drange, vrange) or just simply limit the amount of work done by a single balance run.
The argument may be a single value or a range. The single value N means at most N chunks, equivalent to ..N range syntax. Kernels prior to 4.4 accept only the single value format. The range minimum and maximum are inclusive.
- Balance only block groups which have the given number of stripes. The parameter is a range specified as start..end. Makes sense fo block group profiles that utilize striping, ie. RAID0/10/5/6. The range minimum and maximum are inclusive.
- Takes no parameters. Only has meaning when converting between profiles. When doing convert from one profile to another and soft mode is on, chunks that already have the target profile are left untouched. This is useful e.g. when half of the filesystem was converted earlier but got cancelled.
The soft mode switch is (like every other filter) per-type. For example, this means that we can convert metadata chunks the "hard" way while converting data chunks selectively with soft switch.
Profile names, used in profiles and convert are one of: raid0, raid1, raid10, raid5, raid6, dup, single. The mixed data/metadata profiles can be converted in the same way, but it’s conversion between mixed and non-mixed is not implemented. For the constraints of the profiles please refer to
mkfs.btrfs(8), section PROFILES.
The way balance operates, it usually needs to temporarily create a new block group and move the old data there. For that it needs work space, otherwise it fails for Enospc reasons. This is not the same Enospc as if the free space is exhausted. This refers to the space on the level of block groups.
The free work space can be calculated from the output of the btrfs filesystem show command:
Label: 'BTRFS' uuid: 8a9d72cd-ead3-469d-b371-9c7203276265 Total devices 2 FS bytes used 77.03GiB devid 1 size 53.90GiB used 51.90GiB path /dev/sdc2 devid 2 size 53.90GiB used 51.90GiB path /dev/sde1
size - used = free work space 53.90GiB - 51.90GiB = 2.00GiB
An example of a filter that does not require workspace is usage=0. This will scan through all unused block groups of a given type and will reclaim the space. Ater that it might be possible to run other filters.
CONVERSIONS ON MULTIPLE DEVICES
Conversion to profiles based on striping (RAID0, RAID5/6) require the work space on each device. An interrupted balance may leave partially filled block groups that might consume the work space.
btrfs balance returns a zero exit status if it succeeds. Non zero is returned in case of failure.
btrfs is part of btrfs-progs. Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for further details.