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From "Stefan Podkowinski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-12991) Inter-node race condition in validation compaction
Date Mon, 05 Dec 2016 09:43:58 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-12991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15721759#comment-15721759
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Stefan Podkowinski commented on CASSANDRA-12991:
------------------------------------------------

My assumption is that validation compaction works as follows:
* involved nodes receive a ValidationRequest message
* affected keyspace is being flushed
* validation is started using sstables candidates determined right after the flush

I don't see why you'd have to "SSTables created after that timestamp to be filtered when doing
a validation compaction". Any SSTable created after the validation compaction was started
should not be involved in the validation process anyways. 


> 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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