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BTRFS-BALANCE(8) | Btrfs Manual | BTRFS-BALANCE(8) |
NAME¶
btrfs-balance - balance block groups on a btrfs filesystemSYNOPSIS¶
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 and will be resumed upon mount, unless the mount option skip_balance is specified.•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.
COMPATIBILITY¶
PERFORMANCE IMPLICATIONS¶
Balance operation is intense namely in the IO respect, but can be also CPU intense. It affects other actions on the filesystem. There are typically lots of data being copied from one location to another, and lots of metadata get updated. Depending on the actual block group layout, it can be also seek-heavy. The performance on rotational devices is noticeably worse than on SSDs or fast arrays.SUBCOMMAND¶
cancel <path>cancel running or paused balance, the command will block
and wait until the actually processed blockgroup is 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 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 alwo work and mean no filters.
Options
-d[ <filters>]
status [-v] <path>
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
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 fi 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. For that it needs 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. 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
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 fi 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
•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 fi 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
# btrfs balance start -dusage=85 /path Done, had to relocate 13 out of 95 chunks $ btrfs fi 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
# btrfs balance start -musage=50 /path Done, had to relocate 4 out of 89 chunks $ btrfs fi 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
# btrfs balance start -musage=70 /path Done, had to relocate 13 out of 88 chunks $ btrfs fi 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
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)10/02/2016 | Btrfs v4.7.3 |