402 lines
16 KiB
Plaintext
402 lines
16 KiB
Plaintext
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. Extent sharing is preserved and reflinks are not broken.
|
|
Files are not defragmented nor recompressed, file extents are preserved
|
|
but the physical location on devices will change.
|
|
|
|
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 as an internal state 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
|
|
move data/metadata from the whol filesystem and needs to update all block pointers.
|
|
|
|
The filters can be used to perform following actions:
|
|
|
|
- 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 only the 'system' type needs the force
|
|
option. Otherwise 'system' gets automatically converted whenever 'metadata'
|
|
profile is converted.
|
|
|
|
When metadata redundancy is reduced (eg. from RAID1 to single) the force option
|
|
is also required and it is noted in system log.
|
|
|
|
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 any more.
|
|
|
|
NOTE: A short syntax *btrfs balance <path>* works due to backward compatibility
|
|
but is deprecated and should not be used any more. Use *btrfs balance start*
|
|
command instead.
|
|
|
|
PERFORMANCE IMPLICATIONS
|
|
------------------------
|
|
|
|
Balancing operations are very IO intensive and can also be quite CPU intensive,
|
|
impacting other ongoing filesystem operations. Typically large amounts of data
|
|
are copied from one location to another, with corresponding metadata updates.
|
|
|
|
Depending upon the block group layout, it can also be seek heavy. Performance
|
|
on rotational devices is noticeably worse compared to SSDs or fast arrays.
|
|
|
|
SUBCOMMAND
|
|
----------
|
|
*cancel* <path>::
|
|
cancels a running or paused balance, the command will block and wait until the
|
|
current blockgroup 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, eg. 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 (ie. 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 `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'.
|
|
-f::::
|
|
force a reduction of metadata integrity, eg. 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 `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
|
|
|
|
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 filesystem 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',
|
|
'raid1c3', 'raid1c4', '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, 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.
|
|
|
|
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 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, 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 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 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 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, 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 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.
|
|
|
|
--------------------
|
|
# 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 btrfs wiki http://btrfs.wiki.kernel.org for
|
|
further details.
|
|
|
|
SEE ALSO
|
|
--------
|
|
`mkfs.btrfs`(8),
|
|
`btrfs-device`(8)
|