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63 lines
2.7 KiB
ReStructuredText
63 lines
2.7 KiB
ReStructuredText
The primary purpose of the balance feature is to spread block groups across
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all devices so they match constraints defined by the respective profiles. See
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:doc:`mkfs.btrfs(8)<mkfs.btrfs>` section *PROFILES* for more details.
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The scope of the balancing process can be further tuned by use of filters that
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can select the block groups to process. Balance works only on a mounted
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filesystem. Extent sharing is preserved and reflinks are not broken.
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Files are not defragmented nor recompressed, file extents are preserved
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but the physical location on devices will change.
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The balance operation is cancellable by the user. The on-disk state of the
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filesystem is always consistent so an unexpected interruption (e.g. system crash,
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reboot) does not corrupt the filesystem. The progress of the balance operation
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is temporarily stored as an internal state and will be resumed upon mount,
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unless the mount option *skip_balance* is specified.
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.. warning::
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Running balance without filters will take a lot of time as it basically move
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data/metadata from the whole filesystem and needs to update all block
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pointers.
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The filters can be used to perform following actions:
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- convert block group profiles (filter *convert*)
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- make block group usage more compact (filter *usage*)
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- perform actions only on a given device (filters *devid*, *drange*)
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The filters can be applied to a combination of block group types (data,
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metadata, system). Note that changing only the *system* type needs the force
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option. Otherwise *system* gets automatically converted whenever *metadata*
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profile is converted.
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When metadata redundancy is reduced (e.g. from RAID1 to single) the force option
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is also required and it is noted in system log.
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.. note::
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The balance operation needs enough work space, i.e. space that is completely
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unused in the filesystem, otherwise this may lead to ENOSPC reports. See
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the section *ENOSPC* for more details.
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Compatibility
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-------------
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.. note::
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The balance subcommand also exists under the **btrfs filesystem** namespace.
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This still works for backward compatibility but is deprecated and should not
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be used any more.
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.. note::
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A short syntax **btrfs balance <path>** works due to backward compatibility
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but is deprecated and should not be used any more. Use **btrfs balance start**
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command instead.
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Performance implications
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------------------------
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Balancing operations are very IO intensive and can also be quite CPU intensive,
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impacting other ongoing filesystem operations. Typically large amounts of data
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are copied from one location to another, with corresponding metadata updates.
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Depending upon the block group layout, it can also be seek heavy. Performance
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on rotational devices is noticeably worse compared to SSDs or fast arrays.
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