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NAME | SYNOPSIS | DESCRIPTION | COMPATIBILITY | PERFORMANCE IMPLICATIONS | SUBCOMMAND | FILTERS | ENOSPC | EXAMPLES | EXIT STATUS | AVAILABILITY | SEE ALSO | COLOPHON |
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BTRFS-BALANCE(8) Btrfs Manual BTRFS-BALANCE(8)
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 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.
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.
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.
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
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.
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.
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.
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.
btrfs is part of btrfs-progs. Please refer to the btrfs wiki
http://btrfs.wiki.kernel.org for further details.
mkfs.btrfs(8), btrfs-device(8)
This page is part of the btrfs-progs (btrfs filesystem tools)
project. Information about the project can be found at
⟨https://btrfs.wiki.kernel.org/index.php/Btrfs_source_repositories⟩.
If you have a bug report for this manual page, see
⟨https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#How_do_I_report_bugs_and_issues.3F⟩.
This page was obtained from the project's upstream Git repository
⟨git://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git⟩
on 2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-02.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
Btrfs v5.16.1 02/06/2022 BTRFS-BALANCE(8)
Pages that refer to this page: btrfs(8), btrfs-convert(8), btrfs-device(8), btrfstune(8), mkfs.btrfs(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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