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https://issues.apache.org/jira/browse/CASSANDRA-7409?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14278871#comment-14278871
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Carl Yeksigian commented on CASSANDRA-7409:
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It works in much the same way as was described before; except that:
- When promoting to a level which is not overlapping, the selection will take
all overlapping sstables to compact, rather than just a single one. This is how
it was supposed to work initially, but was actually hitting a different case
instead.
- When selecting the level which needs to be compacted, it starts from 0 going
up to MOLO - 1, then from the max level down to MOLO. This means that we try to
compact any overlapping levels first, and if we don't find anything, then we'll
compact according to our previous ways of compacting.
I'm a little suspicious of the MOLO=0 results, now that I've specified the
changes; it should more closely mirror the results I got from LCS w/o STCS.
It is sensitive to the length of the test; L5 allows for as many overlaps as
necessary in this test. The levels shouldn't have any overlap, though; it won't
stop compactions until none of the levels have overlap. The L2 runs slower
because it is doing more I/O because of the non-overlapping.
> Allow multiple overlapping sstables in L1
> -----------------------------------------
>
> Key: CASSANDRA-7409
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7409
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Carl Yeksigian
> Assignee: Carl Yeksigian
> Labels: compaction
> Fix For: 3.0
>
>
> Currently, when a normal L0 compaction takes place (not STCS), we take up to
> MAX_COMPACTING_L0 L0 sstables and all of the overlapping L1 sstables and
> compact them together. If we didn't have to deal with the overlapping L1
> tables, we could compact a higher number of L0 sstables together into a set
> of non-overlapping L1 sstables.
> This could be done by delaying the invariant that L1 has no overlapping
> sstables. Going from L1 to L2, we would be compacting fewer sstables together
> which overlap.
> When reading, we will not have the same one sstable per level (except L0)
> guarantee, but this can be bounded (once we have too many sets of sstables,
> either compact them back into the same level, or compact them up to the next
> level).
> This could be generalized to allow any level to be the maximum for this
> overlapping strategy.
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