bionic (8) btrfs-balance.8.gz

Provided by: btrfs-progs_4.15.1-1build1_amd64 bug

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 reducing 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 filesytem 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)