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https://issues.apache.org/jira/browse/CASSANDRA-5351?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13822771#comment-13822771
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Jonathan Ellis edited comment on CASSANDRA-5351 at 11/14/13 6:57 PM:
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I think everyone agrees that repaired data should have a leveling system
exactly like current LCS. Freshly repaired sstables (streamed from other
replicas) start in L0 and get leveled from there.
The tricky part is, what do we do about the unrepaired data? The two proposals
are:
# Keep a separate unrepaired arena -- let's call it L0' to distinguish from the
repaired L0 -- and perform STCS in it the way we do with an overflowing L0
currently. Once repaired we move sstables into L0 proper.
# Keep a complete extra set of levels for unrepaired data, L0', L1', ..., LN'
and perform leveling on these, separate from the repaired levels. Once
repaired, data from here will be dropped into repaired L0.
The downside to 1 is that you don't get full leveling benefits until it's
repaired. The downside to 2 is that LCS already has super high write
amplification properties (relative to STCS) so doubling that is going to be
even more painful.
Both downsides get mitigated by repairing more often.
Having written that out ... I'd lean towards the STCS option because it's so
much simpler to implement.
was (Author: jbellis):
I think everyone agrees that repaired data should have a leveling system
exactly like current LCS. Freshly repaired sstables (streamed from other
replicas) start in L0 and get leveled from there.
The tricky part is, what do we do about the unrepaired data? The two proposals
are:
# Keep a separate unrepaired arena -- let's call it L0' to distinguish from the
repaired L0 -- and perform STCS in it the way we do with an overflowing L0
currently. Once repaired we move sstables into L0 proper.
# Keep a complete extra set of levels for unrepaired data, L0', L1', ..., LN'.
Once repaired, data from here will be dropped into repaired L0.
The downside to 1 is that you don't get full leveling benefits until it's
repaired. The downside to 2 is that LCS already has super high write
amplification properties (relative to STCS) so doubling that is going to be
even more painful.
Both downsides get mitigated by repairing more often.
Having written that out ... I'd lean towards the STCS option because it's so
much simpler to implement.
> Avoid repairing already-repaired data by default
> ------------------------------------------------
>
> Key: CASSANDRA-5351
> URL: https://issues.apache.org/jira/browse/CASSANDRA-5351
> Project: Cassandra
> Issue Type: Task
> Components: Core
> Reporter: Jonathan Ellis
> Assignee: Lyuben Todorov
> Labels: repair
> Fix For: 2.1
>
>
> Repair has always built its merkle tree from all the data in a columnfamily,
> which is guaranteed to work but is inefficient.
> We can improve this by remembering which sstables have already been
> successfully repaired, and only repairing sstables new since the last repair.
> (This automatically makes CASSANDRA-3362 much less of a problem too.)
> The tricky part is, compaction will (if not taught otherwise) mix repaired
> data together with non-repaired. So we should segregate unrepaired sstables
> from the repaired ones.
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