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btrfs-progs: docs: add zone reclaim

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Issue: #768
Signed-off-by: David Sterba <dsterba@suse.com>
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David Sterba 2024-07-30 18:14:25 +02:00
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commit fca2fe3316
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Documentation/ch-zoned-intro.rst
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@ -66,6 +66,20 @@ is 8GiB, which would mean that e.g. offset 0-16GiB would be reserved just for
the super block on a hypothetical device of that zone size. This is wasteful
but required to guarantee crash safety.
Zone reclaim, garbage collection
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
As the zones are append-only, overwriting data or COW changes in metadata
make parts of the zones used but not connected to the filesystem structures.
This makes the space unusable and grows over time. Once the ratio hits a
(configurable) threshold a background reclaim process is started and relocates
the remaining blocks in use to a new zone. The old one is reset and can be used
again.
This process may take some time depending on other background work or
amount of new data written. It is possible to hit an intermittent ENOSPC.
Some devices also limit number of active zones.
Devices
^^^^^^^