Manpages - btrfs-balance.8

btrfs-balance - balance block groups on a btrfs filesystem

btrfs balance <subcommand> <args>

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.

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.

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.

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.

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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

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status [-v] <path>

Show status of running or paused balance.

Options

-v

#+begin_quote (deprecated) alias for global -v option

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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.

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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.

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*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.

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 *btrfs-device*(8).

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 btrfs wiki http://btrfs.wiki.kernel.org for further details.

*mkfs.btrfs*(8), *btrfs-device*(8)