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https://issues.apache.org/jira/browse/CASSANDRA-7019?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14307369#comment-14307369
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Marcus Eriksson commented on CASSANDRA-7019:
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What we want is to be able to drop more tombstones by doing a specific
tombstone removal compaction.
To be able to drop as many tombstones as possible, we want to include as many
overlapping sstables as we can in this compaction.
Currently we do this with a single sstable - we find one single sstable,
estimate how many droppable tombstones we have and if more than X% (20 iirc) of
all keys in the sstables are droppable tombstones, we trigger a single sstable
compaction including that. This is often quite ineffective as the tombstones
can cover data in other sstables.
Start by reading up on SizeTieredCompactionStrategy#worthDroppingTombstones()
So, we need to
# Find a good candidate sstable
# Include all sstables that overlap that sstable and contain older data (a
tombstone can only cover older data in other sstables)
# Start a compaction
# Figure out a good way to write out the data to disk (for STCS for example,
all sstables might overlap eachother, which would cause a major compaction, for
LCS we need to distribute the result in the leveled hierarchy somehow). This is
the trickiest part of the ticket. One way I've though about is to track which
sstable the data is coming from and map each input sstable to an output
sstable, and write all non-tombstone data to those. The result would be the
same number of input sstables, minus tombstones (and any covered data)
> Improve tombstone compactions
> -----------------------------
>
> Key: CASSANDRA-7019
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7019
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Marcus Eriksson
> Assignee: Branimir Lambov
> Labels: compaction
> Fix For: 3.0
>
>
> When there are no other compactions to do, we trigger a single-sstable
> compaction if there is more than X% droppable tombstones in the sstable.
> In this ticket we should try to include overlapping sstables in those
> compactions to be able to actually drop the tombstones. Might only be doable
> with LCS (with STCS we would probably end up including all sstables)
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