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https://issues.apache.org/jira/browse/CASSANDRA-9597?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14585527#comment-14585527
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Jeff Jirsa commented on CASSANDRA-9597:
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You can understand why this happens when you realize that the sstables are
filtered by max timestamp:
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/compaction/DateTieredCompactionStrategy.java#L178
And then the resulting list is sorted by min timestamp:
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/compaction/DateTieredCompactionStrategy.java#L357-L367
The result is that for roughly evenly distributed time periods (file size
proportional to sstable maxTimestamp - sstable minTimestamp, which is likely
mostly true for most DTCS workloads), larger files will always be at the front
of {{trimToThreshold}}, which virtually guarantees we'll re-compact a very
large sstable over and over and over if any other sstables are in the window
for compaction.
> DTCS should consider file SIZE in addition to time windowing
> ------------------------------------------------------------
>
> Key: CASSANDRA-9597
> URL: https://issues.apache.org/jira/browse/CASSANDRA-9597
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Jeff Jirsa
> Priority: Minor
> Labels: dtcs
>
> DTCS seems to work well for the typical use case - writing data in perfect
> time order, compacting recent files, and ignoring older files.
> However, there are "normal" operational actions where DTCS will fall behind
> and is unlikely to recover.
> An example of this is streaming operations (for example, bootstrap or loading
> data into a cluster using sstableloader), where lots (tens of thousands) of
> very small sstables can be created spanning multiple time buckets. In these
> case, even if max_sstable_age_days is extended to allow the older incoming
> files to be compacted, the selection logic is likely to re-compact large
> files with fewer small files over and over, rather than prioritizing
> selection of max_threshold smallest files to decrease the number of candidate
> sstables as quickly as possible.
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