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https://issues.apache.org/jira/browse/CASSANDRA-8979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14392508#comment-14392508
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Stefan Podkowinski commented on CASSANDRA-8979:
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Yes, I guess the implications for clusters running mixed versions haven't been
really considered. I've added a new patch which makes sure the digest
calculation for non-empty rows will stay the same after the patch.
Changes in detail:
PrecompactedRow
Would be reverted to exact same code as before the patch.
The issue described in the ticket would still be solved by the patched
Validator, which now only creates a new RowHash in case the digest has been
updated. The PrecompactRow never updated the digest in case of empty rows.
LazilyCompactedRow
I've changed the digest update call condition to: nonEmpty || !DeletionTime.LIVE
this makes sure that digest update is called in any case for non-empty rows
(and will thus be backwards compatible) but still skips update for empty rows
with LIVE deletes.
As for backwards compatability it still should be clear that after the patch
empty/purged rows will now not create a digest in the MT anymore compared to
versions before the patch. In this case those rows would still be streamed
after repairs. This applies to both Precompacted and Lazy. One node with the
new version would create the same mismatch as a node with the old version and a
missing row.
> MerkleTree mismatch for deleted and non-existing rows
> -----------------------------------------------------
>
> Key: CASSANDRA-8979
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8979
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Stefan Podkowinski
> Assignee: Stefan Podkowinski
> Fix For: 2.1.5
>
> Attachments: 8979-AvoidBufferAllocation-2.0_patch.txt,
> 8979-LazilyCompactedRow-2.0.txt, 8979-RevertPrecompactedRow-2.0.txt,
> cassandra-2.0-8979-lazyrow_patch.txt, cassandra-2.0-8979-validator_patch.txt,
> cassandra-2.0-8979-validatortest_patch.txt,
> cassandra-2.1-8979-lazyrow_patch.txt, cassandra-2.1-8979-validator_patch.txt
>
>
> Validation compaction will currently create different hashes for rows that
> have been deleted compared to nodes that have not seen the rows at all or
> have already compacted them away.
> In case this sounds familiar to you, see CASSANDRA-4905 which was supposed to
> prevent hashing of expired tombstones. This still seems to be in place, but
> does not address the issue completely. Or there was a change in 2.0 that
> rendered the patch ineffective.
> The problem is that rowHash() in the Validator will return a new hash in any
> case, whether the PrecompactedRow did actually update the digest or not. This
> will lead to the case that a purged, PrecompactedRow will not change the
> digest, but we end up with a different tree compared to not having rowHash
> called at all (such as in case the row already doesn't exist).
> As an implication, repair jobs will constantly detect mismatches between
> older sstables containing purgable rows and nodes that have already compacted
> these rows. After transfering the reported ranges, the newly created sstables
> will immediately get deleted again during the following compaction. This will
> happen for each repair run over again until the sstable with the purgable row
> finally gets compacted.
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