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https://github.com/kdave/btrfs-progs
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274 lines
10 KiB
ReStructuredText
274 lines
10 KiB
ReStructuredText
btrfs-balance(8)
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================
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SYNOPSIS
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--------
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**btrfs balance** <subcommand> <args>
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DESCRIPTION
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-----------
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.. include:: ch-balance-intro.rst
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SUBCOMMAND
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----------
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cancel <path>
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cancels a running or paused balance, the command will block and wait until the
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current block group being processed completes
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Since kernel 5.7 the response time of the cancellation is significantly
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improved, on older kernels it might take a long time until currently
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processed chunk is completely finished.
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pause <path>
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pause running balance operation, this will store the state of the balance
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progress and used filters to the filesystem
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resume <path>
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resume interrupted balance, the balance status must be stored on the filesystem
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from previous run, e.g. after it was paused or forcibly interrupted and mounted
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again with *skip_balance*
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start [options] <path>
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start the balance operation according to the specified filters, without any filters
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the data and metadata from the whole filesystem are moved. The process runs in
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the foreground.
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.. note::
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The balance command without filters will basically move everything in the
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filesystem to a new physical location on devices (i.e. it does not affect the
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logical properties of file extents like offsets within files and extent
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sharing). The run time is potentially very long, depending on the filesystem
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size. To prevent starting a full balance by accident, the user is warned and
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has a few seconds to cancel the operation before it starts. The warning and
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delay can be skipped with *--full-balance* option.
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Please note that the filters must be written together with the *-d*, *-m* and
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*-s* options, because they're optional and bare *-d* and *-m* also work and
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mean no filters.
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.. note::
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When the target profile for conversion filter is *raid5* or *raid6*,
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there's a safety timeout of 10 seconds to warn users about the status of the feature
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``Options``
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-d[<filters>]
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act on data block groups, see *FILTERS* section for details about *filters*
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-m[<filters>]
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act on metadata chunks, see *FILTERS* section for details about *filters*
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-s[<filters>]
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act on system chunks (requires *-f*), see *FILTERS* section for details about *filters*.
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-f
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force a reduction of metadata integrity, e.g. when going from *raid1* to
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*single*, or skip safety timeout when the target conversion profile is *raid5*
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or *raid6*
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--background|--bg
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run the balance operation asynchronously in the background, uses ``fork(2)`` to
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start the process that calls the kernel ioctl
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--enqueue
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wait if there's another exclusive operation running, otherwise continue
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-v
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(deprecated) alias for global '-v' option
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status [-v] <path>
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Show status of running or paused balance.
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``Options``
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-v
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(deprecated) alias for global *-v* option
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FILTERS
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-------
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.. include:: ch-balance-filters.rst
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ENOSPC
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------
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The way balance operates, it usually needs to temporarily create a new block
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group and move the old data there, before the old block group can be removed.
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For that it needs the work space, otherwise it fails for ENOSPC reasons.
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This is not the same ENOSPC as if the free space is exhausted. This refers to
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the space on the level of block groups, which are bigger parts of the filesystem
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that contain many file extents.
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The free work space can be calculated from the output of the **btrfs filesystem show**
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command:
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.. code-block:: none
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Label: 'BTRFS' uuid: 8a9d72cd-ead3-469d-b371-9c7203276265
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Total devices 2 FS bytes used 77.03GiB
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devid 1 size 53.90GiB used 51.90GiB path /dev/sdc2
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devid 2 size 53.90GiB used 51.90GiB path /dev/sde1
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*size* - *used* = *free work space*
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*53.90GiB* - *51.90GiB* = *2.00GiB*
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An example of a filter that does not require workspace is *usage=0*. This will
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scan through all unused block groups of a given type and will reclaim the
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space. After that it might be possible to run other filters.
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**CONVERSIONS ON MULTIPLE DEVICES**
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Conversion to profiles based on striping (RAID0, RAID5/6) require the work
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space on each device. An interrupted balance may leave partially filled block
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groups that consume the work space.
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EXAMPLES
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--------
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A more comprehensive example when going from one to multiple devices, and back,
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can be found in section *TYPICAL USECASES* of :doc:`btrfs-device(8)<btrfs-device>`.
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MAKING BLOCK GROUP LAYOUT MORE COMPACT
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The layout of block groups is not normally visible; most tools report only
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summarized numbers of free or used space, but there are still some hints
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provided.
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Let's use the following real life example and start with the output:
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.. code-block:: none
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$ btrfs filesystem df /path
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Data, single: total=75.81GiB, used=64.44GiB
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System, RAID1: total=32.00MiB, used=20.00KiB
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Metadata, RAID1: total=15.87GiB, used=8.84GiB
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GlobalReserve, single: total=512.00MiB, used=0.00B
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Roughly calculating for data, *75G - 64G = 11G*, the used/total ratio is
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about *85%*. How can we can interpret that:
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* chunks are filled by 85% on average, i.e. the *usage* filter with anything
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smaller than 85 will likely not affect anything
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* in a more realistic scenario, the space is distributed unevenly, we can
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assume there are completely used chunks and the remaining are partially filled
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Compacting the layout could be used on both. In the former case it would spread
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data of a given chunk to the others and removing it. Here we can estimate that
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roughly 850 MiB of data have to be moved (85% of a 1 GiB chunk).
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In the latter case, targeting the partially used chunks will have to move less
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data and thus will be faster. A typical filter command would look like:
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.. code-block:: none
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# btrfs balance start -dusage=50 /path
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Done, had to relocate 2 out of 97 chunks
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$ btrfs filesystem df /path
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Data, single: total=74.03GiB, used=64.43GiB
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System, RAID1: total=32.00MiB, used=20.00KiB
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Metadata, RAID1: total=15.87GiB, used=8.84GiB
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GlobalReserve, single: total=512.00MiB, used=0.00B
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As you can see, the *total* amount of data is decreased by just 1 GiB, which is
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an expected result. Let's see what will happen when we increase the estimated
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usage filter.
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.. code-block:: none
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# btrfs balance start -dusage=85 /path
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Done, had to relocate 13 out of 95 chunks
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$ btrfs filesystem df /path
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Data, single: total=68.03GiB, used=64.43GiB
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System, RAID1: total=32.00MiB, used=20.00KiB
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Metadata, RAID1: total=15.87GiB, used=8.85GiB
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GlobalReserve, single: total=512.00MiB, used=0.00B
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Now the used/total ratio is about 94% and we moved about *74G - 68G = 6G* of
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data to the remaining block groups, i.e. the 6GiB are now free of filesystem
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structures, and can be reused for new data or metadata block groups.
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We can do a similar exercise with the metadata block groups, but this should
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not typically be necessary, unless the used/total ratio is really off. Here
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the ratio is roughly 50% but the difference as an absolute number is "a few
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gigabytes", which can be considered normal for a workload with snapshots or
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reflinks updated frequently.
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.. code-block:: none
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# btrfs balance start -musage=50 /path
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Done, had to relocate 4 out of 89 chunks
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$ btrfs filesystem df /path
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Data, single: total=68.03GiB, used=64.43GiB
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System, RAID1: total=32.00MiB, used=20.00KiB
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Metadata, RAID1: total=14.87GiB, used=8.85GiB
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GlobalReserve, single: total=512.00MiB, used=0.00B
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Just 1 GiB decrease, which possibly means there are block groups with good
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utilization. Making the metadata layout more compact would in turn require
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updating more metadata structures, i.e. lots of IO. As running out of metadata
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space is a more severe problem, it's not necessary to keep the utilization
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ratio too high. For the purpose of this example, let's see the effects of
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further compaction:
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.. code-block:: none
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# btrfs balance start -musage=70 /path
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Done, had to relocate 13 out of 88 chunks
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$ btrfs filesystem df .
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Data, single: total=68.03GiB, used=64.43GiB
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System, RAID1: total=32.00MiB, used=20.00KiB
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Metadata, RAID1: total=11.97GiB, used=8.83GiB
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GlobalReserve, single: total=512.00MiB, used=0.00B
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GETTING RID OF COMPLETELY UNUSED BLOCK GROUPS
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Normally the balance operation needs a work space, to temporarily move the
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data before the old block groups gets removed. If there's no work space, it
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ends with *no space left*.
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There's a special case when the block groups are completely unused, possibly
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left after removing lots of files or deleting snapshots. Removing empty block
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groups is automatic since 3.18. The same can be achieved manually with a
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notable exception that this operation does not require the work space. Thus it
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can be used to reclaim unused block groups to make it available.
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.. code-block:: bash
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# btrfs balance start -dusage=0 /path
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This should lead to decrease in the *total* numbers in the **btrfs filesystem df** output.
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EXIT STATUS
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-----------
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Unless indicated otherwise below, all **btrfs balance** subcommands
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return a zero exit status if they succeed, and non zero in case of
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failure.
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The **pause**, **cancel**, and **resume** subcommands exit with a status of
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**2** if they fail because a balance operation was not running.
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The **status** subcommand exits with a status of **0** if a balance
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operation is not running, **1** if the command-line usage is incorrect
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or a balance operation is still running, and **2** on other errors.
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AVAILABILITY
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------------
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**btrfs** is part of btrfs-progs. Please refer to the documentation at
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https://btrfs.readthedocs.io or wiki http://btrfs.wiki.kernel.org for further
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information.
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SEE ALSO
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--------
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:doc:`mkfs.btrfs(8)<mkfs.btrfs>`,
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:doc:`btrfs-device(8)<btrfs-device>`
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