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19 lines
780 B
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19 lines
780 B
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Caching
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~~~~~~~
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When an extent has multiple copies on different devices, some of those
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copies may be marked as cached. Buckets containing only cached data are
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discarded as needed by the allocator in LRU order.
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| When data is moved from one device to another according to the
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| ``background_target`` option, the original copy is left in place but
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marked as cached. With the ``promote_target`` option, the original
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copy is left unchanged and the new copy on the ``promote_target``
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device is marked as cached.
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To do writeback caching, set ``foreground_target`` and
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``promote_target`` to the cache device, and ``background_target`` to the
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backing device. To do writearound caching, set ``foreground_target`` to
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the backing device and ``promote_target`` to the cache device.
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