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 :doc:`mkfs.btrfs` section :ref:`PROFILES<man-mkfs-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.
Warning
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.
Note
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.
Note
The balance subcommand also exists under the :command:`btrfs filesystem` namespace. This still works for backward compatibility but is deprecated and should not be used any more.
Note
A short syntax :command:`btrfs balance <path>` works due to backward compatibility but is deprecated and should not be used any more. Use :command:`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.