btrfs balance <subcommand> <args>
- 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 filesystem 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 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 section :ref:`FILTERS<man-balance-filters>` for details about filters
- -m[<filters>]
- act on metadata chunks, see :ref:`FILTERS<man-balance-filters>` for details about filters
- -s[<filters>]
- act on system chunks (requires -f), see :ref:`FILTERS<man-balance-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 :manref:`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
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 :doc:`btrfs-device`.
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 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 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. Making 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 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
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 /pathThis 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 documentation at https://btrfs.readthedocs.io.