BTRFS-BALANCE(8) Btrfs Manual 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.
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.
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:
o convert block group profiles (filter convert)
o make block group usage more compact (filter usage)
o 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 anymore.
Note
A short syntax btrfs balance <path> works due to backward
compatibility but is deprecated and should not be used anymore. Use
btrfs balance start command instead.
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>]
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 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
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 might 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 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
Roughly calculating for data, 75G - 64G = 11G, the used/total ratio is
about 85%. How can we can interpret that:
o chunks are filled by 85% on average, ie. the usage filter with
anything smaller than 85 will likely not affect anything
o 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
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 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
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 be typically necessary, unless the used/total ration 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 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
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 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
This should lead to decrease in the total numbers in the btrfs fi 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)
Btrfs v4.9.1 08/06/2017 BTRFS-BALANCE(8)
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