Ubuntu Manpage: btrfs-balance - balance block groups on a btrfs filesystem

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. Extent sharing is preserved and reflinks are not broken. Files are not
defragmented nor recompressed, file extents are preserved but the physical location on
devices will change.

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 move
data/metadata from the whol 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 only the system type needs the force option. Otherwise system gets
automatically converted whenever metadata profile is converted.

When metadata redundancy is reduced (eg. from RAID1 to single) the force option is also
required and it is noted in system log.

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

           Since kernel 5.7 the response time of the cancellation is significantly improved, on
           older kernels it might take a long time until currently processed chunk is completely
           finished.

       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 paused or forcibly interrupted and mounted again with
           skip_balance

       start [options] <path>
           start the balance operation according to the specified filters, without any filters
           the data and metadata from the whole filesystem are moved. The process runs in the
           foreground.

               Note
               the balance command without filters will basically move everything in the
               filesystem to a new physical location on devices (ie. it does not affect the
               logical properties of file extents like offsets within files and extent sharing).
               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 and -m also work and mean no filters.

               Note
               when the target profile for conversion filter is raid5 or raid6, there’s a safety
               timeout of 10 seconds to warn users about the status of the feature
           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.

           -f
               force a reduction of metadata integrity, eg. when going from raid1 to single, or
               skip safety timeout when the target conversion profile is raid5 or raid6

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

           --enqueue
               wait if there’s another exclusive operation running, otherwise continue

           -v
               (deprecated) alias for global -v option

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

           Options

           -v
               (deprecated) alias for global -v option

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, raid1c3, raid1c4,
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

       Unless indicated otherwise below, all btrfs balance subcommands return a zero exit status
       if they succeed, and non zero in case of failure.

       The pause, cancel, and resume subcommands exit with a status of 2 if they fail because a
       balance operation was not running.

       The status subcommand exits with a status of 0 if a balance operation is not running, 1 if
       the command-line usage is incorrect or a balance operation is still running, and 2 on
       other errors.

AVAILABILITY

       btrfs is part of btrfs-progs. Please refer to the btrfs wiki http://btrfs.wiki.kernel.org
       for further details.