NAME
btrfs-balance - balance block groups on a btrfs filesystemSYNOPSIS
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.Running balance without filters will take a
lot of time as it basically move data/metadata from the whole filesystem and
needs to update all block pointers.
- •
- 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 balance operation needs enough work space,
i.e. 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.
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.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.
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
- -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, 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 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.- 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.
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, i.e. 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.
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.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
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.$ 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
- •
- 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
# 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
# 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
# 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
# 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.# btrfs balance start -dusage=0 /path
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.AVAILABILITY
btrfs is part of btrfs-progs. Please refer to the documentation at https://btrfs.readthedocs.io or wiki http://btrfs.wiki.kernel.org for further information.SEE ALSO
mkfs.btrfs(8), btrfs-device(8)February 28, 2023 | 6.2 |