BTRFS-BALANCE(8)                      Btrfs Manual                     BTRFS-BALANCE(8)

NAME
       btrfs-balance - balance block groups on a btrfs filesystem

SYNOPSIS
       btrfs balance <subcommand> <args>

DESCRIPTION
       The primary purpose of the balance feature is to spread block groups across 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 as an internal state and will be resumed upon mount,
       unless the mount option skip_balance is specified.

           Warning
           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.

           Note
           the balance operation needs enough work space, ie. space that is completely
           unused in the filesystem, otherwise this may lead to ENOSPC reports. See the
           section ENOSPC for more details.

COMPATIBILITY
           Note
           The balance subcommand also exists under the btrfs filesystem namespace.
           This still works for backward compatibility but is deprecated and should not
           be used any more.

           Note
           A short syntax btrfs balance <path> works due to backward compatibility but
           is deprecated and should not be used any more. Use btrfs balance start
           command instead.

PERFORMANCE IMPLICATIONS
       Balancing operations are very IO intensive and can also be quite CPU intensive,
       impacting other ongoing filesystem operations. Typically large amounts of data
       are copied from one location to another, with corresponding metadata updates.

       Depending upon the block group layout, it can also be seek heavy. Performance on
       rotational devices is noticeably worse compared to SSDs or fast arrays.

SUBCOMMAND
       cancel <path>
           cancels a running or paused balance, the command will block and wait until
           the current blockgroup being processed completes

       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, the balance status must be stored on the
           filesystem from previous run, eg. after it was forcibly interrupted and
           mounted again with skip_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.

               Note
               the balance command without filters will basically rewrite everything in
               the filesystem. The run time is potentially very long, depending on the
               filesystem size. To prevent starting a full balance by accident, the
               user is warned and has a few seconds to cancel the operation before it
               starts. The warning and delay can be skipped with --full-balance option.
           Please note that the filters must be written together with the -d, -m and -s
           options, because they’re optional and bare -d etc also work and mean no
           filters.

           Options

           -d[<filters>]
               act on data block groups, see FILTERS section for details about filters

           -m[<filters>]
               act on metadata chunks, see FILTERS section for details about filters

           -s[<filters>]
               act on system chunks (requires -f), see FILTERS section for details
               about filters.

           -v
               be verbose and print balance filter arguments

           -f
               force a reduction of metadata integrity, eg. when going from raid1 to
               single

           --background|--bg
               run the balance operation asynchronously in the background, uses fork(2)
               to start the process that calls the kernel ioctl

       status [-v] <path>
           Show status of running or paused balance.

           If -v option is given, output will be verbose.

FILTERS
       From kernel 3.3 onwards, btrfs balance can limit its action to a subset of the
       whole 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 structure: type[=params][,type=...]

       The available types are:

       profiles=<profiles>
           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 returning 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.

       devid=<id>
           Balances only block groups which have at least one chunk on the given
           device. To list devices with ids use btrfs filesystem show.

       drange=<range>
           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.

       vrange=<range>
           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=<profile>
           Convert each selected block group to the given profile name identified by
           parameters.

               Note
               starting with kernel 4.5, the data chunks can be converted to/from the
               DUP profile on a single device.

               Note
               starting with kernel 4.6, all profiles can be converted to/from DUP on
               multi-device filesystems.

       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.

       stripes=<range>
           Balance only block groups which have the given number of stripes. The
           parameter is a range specified as start..end. Makes sense for block group
           profiles that utilize striping, ie. RAID0/10/5/6. The range minimum and
           maximum are inclusive.

       soft
           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.

ENOSPC
       The way balance operates, it usually needs to temporarily create a new block
       group and move the old data there, before the old block group can be removed.
       For that it needs the 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, which are bigger parts of the filesystem that
       contain many file extents.

       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.
       After 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 consume the work space.

EXAMPLES
       A more comprehensive example when going from one to multiple devices, and back,
       can be found in section TYPICAL USECASES of btrfs-device(8).

   MAKING BLOCK GROUP LAYOUT MORE COMPACT
       The layout of block groups is not normally visible; most tools report only
       summarized numbers of free or used space, but there are still some hints
       provided.

       Let’s use the following real life example and start with the output:

           $ btrfs filesystem df /path
           Data, single: total=75.81GiB, used=64.44GiB
           System, RAID1: total=32.00MiB, used=20.00KiB
           Metadata, RAID1: total=15.87GiB, used=8.84GiB
           GlobalReserve, single: total=512.00MiB, used=0.00B

       Roughly calculating for data, 75G - 64G = 11G, the used/total ratio is about
       85%. How can we can interpret that:

       •   chunks are filled by 85% on average, ie. the usage filter with anything
           smaller than 85 will likely not affect anything

       •   in a more realistic scenario, the space is distributed unevenly, we can
           assume there are completely used chunks and the remaining are partially
           filled

       Compacting the layout could be used on both. In the former case it would spread
       data of a given chunk to the others and removing it. Here we can estimate that
       roughly 850 MiB of data have to be moved (85% of a 1 GiB chunk).

       In the latter case, targeting the partially used chunks will have to move less
       data and thus will be faster. A typical filter command would look like:

           # btrfs balance start -dusage=50 /path
           Done, had to relocate 2 out of 97 chunks

           $ btrfs filesystem df /path
           Data, single: total=74.03GiB, used=64.43GiB
           System, RAID1: total=32.00MiB, used=20.00KiB
           Metadata, RAID1: total=15.87GiB, used=8.84GiB
           GlobalReserve, single: total=512.00MiB, used=0.00B

       As you can see, the total amount of data is decreased by just 1 GiB, which is an
       expected result. Let’s see what will happen when we increase the estimated usage
       filter.

           # btrfs balance start -dusage=85 /path
           Done, had to relocate 13 out of 95 chunks

           $ btrfs filesystem df /path
           Data, single: total=68.03GiB, used=64.43GiB
           System, RAID1: total=32.00MiB, used=20.00KiB
           Metadata, RAID1: total=15.87GiB, used=8.85GiB
           GlobalReserve, single: total=512.00MiB, used=0.00B

       Now the used/total ratio is about 94% and we moved about 74G - 68G = 6G of data
       to the remaining blockgroups, ie. the 6GiB are now free of filesystem
       structures, and can be reused for new data or metadata block groups.

       We can do a similar exercise with the metadata block groups, but this should not
       typically be necessary, unless the used/total ratio is really off. Here the
       ratio is roughly 50% but the difference as an absolute number is "a few
       gigabytes", which can be considered normal for a workload with snapshots or
       reflinks updated frequently.

           # btrfs balance start -musage=50 /path
           Done, had to relocate 4 out of 89 chunks

           $ btrfs filesystem df /path
           Data, single: total=68.03GiB, used=64.43GiB
           System, RAID1: total=32.00MiB, used=20.00KiB
           Metadata, RAID1: total=14.87GiB, used=8.85GiB
           GlobalReserve, single: total=512.00MiB, used=0.00B

       Just 1 GiB decrease, which possibly means there are block groups with good
       utilization. Making the metadata layout more compact would in turn require
       updating more metadata structures, ie. lots of IO. As running out of metadata
       space is a more severe problem, it’s not necessary to keep the utilization ratio
       too high. For the purpose of this example, let’s see the effects of further
       compaction:

           # btrfs balance start -musage=70 /path
           Done, had to relocate 13 out of 88 chunks

           $ btrfs filesystem df .
           Data, single: total=68.03GiB, used=64.43GiB
           System, RAID1: total=32.00MiB, used=20.00KiB
           Metadata, RAID1: total=11.97GiB, used=8.83GiB
           GlobalReserve, single: total=512.00MiB, used=0.00B

   GETTING RID OF COMPLETELY UNUSED BLOCK GROUPS
       Normally the balance operation needs a work space, to temporarily move the data
       before the old block groups gets removed. If there’s no work space, it ends with
       no space left.

       There’s a special case when the block groups are completely unused, possibly
       left after removing lots of files or deleting snapshots. Removing empty block
       groups is automatic since 3.18. The same can be achieved manually with a notable
       exception that this operation does not require the work space. Thus it can be
       used to reclaim unused block groups to make it available.

           # btrfs balance start -dusage=0 /path

       This should lead to decrease in the total numbers in the btrfs filesystem df
       output.

EXIT STATUS
       btrfs balance 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-device(8)

Btrfs v4.20.1                          01/23/2019                      BTRFS-BALANCE(8)

usage: btrfs balance <command> [options] <path>
   or: btrfs balance <path>

    btrfs balance start [options] <path>
        Balance chunks across the devices
    btrfs balance pause <path>
        Pause running balance
    btrfs balance cancel <path>
        Cancel running or paused balance
    btrfs balance resume <path>
        Resume interrupted balance
    btrfs balance status [-v] <path>
        Show status of running or paused balance

balance data across devices, or change block groups using filters