2021-11-30 15:00:48 +00:00
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Reflink
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=======
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2021-12-09 19:46:42 +00:00
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Reflink is a type of shallow copy of file data that shares the blocks but
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otherwise the files are independent and any change to the file will not affect
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the other. This builds on the underlying COW mechanism. A reflink will
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effectively create only a separate metadata pointing to the shared blocks which
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is typically much faster than a deep copy of all blocks.
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The reflink is typically meant for whole files but a partial file range can be
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also copied, though there are no ready-made tools for that.
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.. code-block:: shell
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cp --reflink=always source target
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2021-12-17 09:49:39 +00:00
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There are some constraints:
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2021-12-09 19:46:42 +00:00
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- cross-filesystem reflink is not possible, there's nothing in common between
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so the block sharing can't work
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2022-06-16 10:56:56 +00:00
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- reflink crossing two mount points of the same filesystem support depends on
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kernel version:
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- until 5.17 it's not supported and fails with "Cross device link", can be
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worked around by performing the operation on the toplevel subvolume
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- works since 5.18
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2021-12-09 19:46:42 +00:00
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- reflink requires source and target file that have the same status regarding
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NOCOW and checksums, for example if the source file is NOCOW (once created
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with the chattr +C attribute) then the above command won't work unless the
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target file is pre-created with the +C attribute as well, or the NOCOW
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attribute is inherited from the parent directory (chattr +C on the directory)
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or if the whole filesystem is mounted with *-o nodatacow* that would create
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the NOCOW files by default
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