275 lines
10 KiB
ReStructuredText
275 lines
10 KiB
ReStructuredText
btrfs-balance(8)
|
|
================
|
|
|
|
SYNOPSIS
|
|
--------
|
|
|
|
**btrfs balance** <subcommand> <args>
|
|
|
|
DESCRIPTION
|
|
-----------
|
|
|
|
.. include:: ch-balance-intro.rst
|
|
|
|
SUBCOMMAND
|
|
----------
|
|
|
|
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
|
|
|
|
.. _man-balance-filters:
|
|
|
|
FILTERS
|
|
-------
|
|
|
|
.. include:: ch-balance-filters.rst
|
|
|
|
ENOSPC
|
|
------
|
|
|
|
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:
|
|
|
|
.. code-block:: none
|
|
|
|
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.
|
|
|
|
EXAMPLES
|
|
--------
|
|
|
|
A more comprehensive example when going from one to multiple devices, and back,
|
|
can be found in section *TYPICAL USECASES* of :doc:`btrfs-device`.
|
|
|
|
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:
|
|
|
|
.. code-block:: none
|
|
|
|
$ 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:
|
|
|
|
.. code-block:: none
|
|
|
|
# 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.
|
|
|
|
.. code-block:: none
|
|
|
|
# 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.
|
|
|
|
.. code-block:: none
|
|
|
|
# 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:
|
|
|
|
.. code-block:: none
|
|
|
|
# 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.
|
|
|
|
.. code-block:: bash
|
|
|
|
# btrfs balance start -dusage=0 /path
|
|
|
|
This should lead to decrease in the *total* numbers in the **btrfs filesystem df** output.
|
|
|
|
EXIT STATUS
|
|
-----------
|
|
|
|
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.
|
|
|
|
AVAILABILITY
|
|
------------
|
|
|
|
**btrfs** is part of btrfs-progs. Please refer to the documentation at
|
|
`https://btrfs.readthedocs.io <https://btrfs.readthedocs.io>`_.
|
|
|
|
SEE ALSO
|
|
--------
|
|
|
|
:doc:`mkfs.btrfs`,
|
|
:doc:`btrfs-device`
|