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BTRFS-BALANCE(8) BTRFS 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. Ex‐
tent sharing is preserved and reflinks are not broken. Files are not defragmented nor recom‐
pressed, 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 al‐
ways consistent so an unexpected interruption (e.g. 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 whole 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 auto‐
matically converted whenever metadata profile is converted.

When metadata redundancy is reduced (e.g. from RAID1 to single) the force option is also re‐
quired and it is noted in system log.

NOTE:
The balance operation needs enough work space, i.e. 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 lo‐
cation 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
block group 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, e.g. 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 filesys‐
tem to a new physical location on devices (i.e. 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 start‐
ing 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 section FILTERS for details about filters

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

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

-f force a reduction of metadata integrity, e.g. 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 filesys‐
tem, and can be used to change the replication configuration (e.g. convert data from single
to RAID1).

Balance can be limited to a block group profile with the following options:

• -d for data block groups

• -m for metadata block groups (also implicitly applies to -s)

• -s for system block groups

The options have an optional parameter which means that the parameter must start right after
the option without a space (this is mandatory getopt syntax), like -dusage=10. Options for
all block group types can be specified in one command.

A filter has the following structure: filter[=params][,filter=...]

To combine multiple filters use ,, without spaces. Example: -dconvert=raid1,soft

BTRFS can have different profiles on a single device or the same profile on multiple device.

The main reason why you want to have different profiles for data and metadata is to provide
additional protection of the filesystem's metadata when devices fail, since a single sector
of unrecoverable metadata will break the filesystem, while a single sector of lost data can
be trivially recovered by deleting the broken file.

Before changing profiles, make sure there is enough unallocated space on existing drives to
create new metadata block groups (for filesystems over 50GiB, this is 1GB * (number_of_de‐
vices + 2)).

Default profiles on BTRFS are:

• data: single

•

metadata:

• single devices: dup

• multiple devices: raid1

The available filter types are:

Filter types
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 per‐
cent 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, i.e. RAID0/10/5/6. The range minimum and maximum are inclusive.

soft Takes no parameters. Only has meaning when converting between profiles, or 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 selec‐
tively 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 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 possi‐
ble to run other filters.

CONVERSIONS ON MULTIPLE DEVICES

Conversion to profiles based on striping (RAID0, RAID5/6) require the work space on each de‐
vice. 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, i.e. 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 re‐
sult. 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 re‐
maining block groups, i.e. 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. Mak‐
ing the metadata layout more compact would in turn require updating more metadata structures,
i.e. lots of IO. As running out of metadata space is a more severe problem, it's not neces‐
sary 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 re‐
moving 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 avail‐
able.

# 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 bal‐
ance 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 documentation at
https://btrfs.readthedocs.io.