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31 lines
1.6 KiB
ReStructuredText
31 lines
1.6 KiB
ReStructuredText
Defragmentation
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===============
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Defragmentation of files is supposed to make the layout of the file extents to
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be more linear or at least coalesce the file extents into larger ones that can
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be stored on the device more efficiently. The reason there's a need for
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defragmentation stems from the COW design that BTRFS is built on and is
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inherent. The fragmentation is caused by rewrites of the same file data
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in-place, that has to be handled by creating a new copy that may lie on a
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distant location on the physical device. Fragmentation is the worst problem on
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rotational hard disks due to the delay caused by moving the drive heads to the
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distant location. With the modern seek-less devices it's not a problem though
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it may still make sense because of reduced size of the metadata that's needed
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to track the scattered extents.
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File data that are in use can be safely defragmented because the whole process
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happens inside the page cache, that is the central point caching the file data
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and takes care of synchronization. Once a filesystem sync or flush is started
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(either manually or automatically) all the dirty data get written to the
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devices. This however reduces the chances to find optimal layout as the writes
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happen together with other data and the result depends on the remaining free
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space layout and fragmentation.
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.. warning::
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Defragmentation does not preserve extent sharing, eg. files created by **cp
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--reflink** or existing on multiple snapshots. Due to that the data space
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consumption may increase.
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Defragmentation can be started together with compression on the given range,
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and takes precedence over per-file compression property or mount options.
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