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https://issues.apache.org/jira/browse/CASSANDRA-12991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15722046#comment-15722046
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Stefan Podkowinski commented on CASSANDRA-12991:
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Let's assume it would be possible to do validation of sstables based on a
provided timestamp, there's still the issue that data could be reconciled in
memtables in a way that would still result in a mismatch.
t = 10000
NodeA - Mutation(k=1, ts=1)
NodeA - Flush
NodeB - Mutation(k=1, ts=1)
t = 10001
NodeA - Mutation(k=1, ts=2)
NodeB - Mutation(k=1, ts=2)
NodeB - Flush
If you start validation based on t(10000) you'd still end up with a mismatch,
as only Mutation(k=1, ts=2) would have been flushed to disk on NodeB, while
ts=1 (which would be subject to the validation timestamp) would not.
> Inter-node race condition in validation compaction
> --------------------------------------------------
>
> Key: CASSANDRA-12991
> URL: https://issues.apache.org/jira/browse/CASSANDRA-12991
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Benjamin Roth
> Priority: Minor
>
> Problem:
> When a validation compaction is triggered by a repair it may happen that due
> to flying in mutations the merkle trees differ but the data is consistent
> however.
> Example:
> t = 10000:
> Repair starts, triggers validations
> Node A starts validation
> t = 10001:
> Mutation arrives at Node A
> t = 10002:
> Mutation arrives at Node B
> t = 10003:
> Node B starts validation
> Hashes of node A+B will differ but data is consistent from a view (think of
> it like a snapshot) t = 10000.
> Impact:
> Unnecessary streaming happens. This may not a big impact on low traffic CFs,
> partitions but on high traffic CFs and maybe very big partitions, this may
> have a bigger impact and is a waste of resources.
> Possible solution:
> Build hashes based upon a snapshot timestamp.
> This requires SSTables created after that timestamp to be filtered when doing
> a validation compaction:
> - Cells with timestamp > snapshot time have to be removed
> - Tombstone range markers have to be handled
> - Bounds have to be removed if delete timestamp > snapshot time
> - Boundary markers have to be either changed to a bound or completely
> removed, depending if start and/or end are both affected or not
> Probably this is a known behaviour. Have there been any discussions about
> this in the past? Did not find an matching issue, so I created this one.
> I am happy about any feedback, whatsoever.
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