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 (e.g. 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.
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.
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 (e.g. from RAID1 to single) the force option is also required and it is noted in system log.
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.
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.
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.
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:
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.
Balances only block groups which have at least one chunk on the given device. To list devices with ids use btrfs filesystem show.
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.
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 each selected block group to the given profile name identified by parameters.
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.
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.
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.