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When receiving a DRR_OBJECT record the receive_object() function needs to determine how to handle a spill block associated with the object. It may need to be removed or kept depending on how the object was modified at the source. This determination is currently accomplished using a heuristic which takes in to account the DRR_OBJECT record and the existing object properties. This is a problem because there isn't quite enough information available to do the right thing under all circumstances. For example, when only the block size changes the spill block is removed when it should be kept. What's needed to resolve this is an additional flag in the DRR_OBJECT which indicates if the object being received references a spill block. The DRR_OBJECT_SPILL flag was added for this purpose. When set then the object references a spill block and it must be kept. Either it is update to date, or it will be replaced by a subsequent DRR_SPILL record. Conversely, if the object being received doesn't reference a spill block then any existing spill block should always be removed. Since previous versions of ZFS do not understand this new flag additional DRR_SPILL records will be inserted in to the stream. This has the advantage of being fully backward compatible. Existing ZFS systems receiving this stream will recreate the spill block if it was incorrectly removed. Updated ZFS versions will correctly ignore the additional spill blocks which can be identified by checking for the DRR_SPILL_UNMODIFIED flag. The small downside to this approach is that is may increase the size of the stream and of the received snapshot on previous versions of ZFS. Additionally, when receiving streams generated by previous unpatched versions of ZFS spill blocks may still be lost. OpenZFS-issue: https://www.illumos.org/issues/9952 FreeBSD-issue: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=233277 Reviewed-by: Paul Dagnelie <pcd@delphix.com> Reviewed-by: Matt Ahrens <mahrens@delphix.com> Reviewed-by: Tom Caputi <tcaputi@datto.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #8668 |
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