[jira] [Updated] (CASSANDRA-12526) For LCS, single SSTable up-level is handled inefficiently

["Wei Deng (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-12526?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Wei Deng updated CASSANDRA-12526:
---------------------------------
    Labels: compaction lcs performance  (was: )

> For LCS, single SSTable up-level is handled inefficiently
> ---------------------------------------------------------
>
>                 Key: CASSANDRA-12526
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12526
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Compaction
>            Reporter: Wei Deng
>              Labels: compaction, lcs, performance
>
> I'm using the latest trunk (as of August 2016, which probably is going to be 3.10) to run some experiments on LeveledCompactionStrategy and noticed this inefficiency.
> The test data is generated using cassandra-stress default parameters (keyspace1.standard1), so as you can imagine, it consists of a ton of newly inserted partitions that will never merge in compactions, which is probably the worst kind of workload for LCS (however, I'll detail later why this scenario should not be ignored as a corner case; for now, let's just assume we still want to handle this scenario efficiently).
> After the compaction test is done, I scrubbed debug.log for patterns that match  the "Compacted" summary so that I can see how long each individual compaction took and how many bytes they processed. The search pattern is like the following:
> {noformat}
> grep 'Compacted.*standard1' debug.log
> {noformat}
> Interestingly, I noticed a lot of the finished compactions are marked as having *only one* SSTable involved. With the workload mentioned above, the "single SSTable" compactions actually consist of the majority of all compactions (as shown below), so its efficiency can affect the overall compaction throughput quite a bit.
> {noformat}
> automaton@0ce59d338-1:~/cassandra-trunk/logs$ grep 'Compacted.*standard1' debug.log-test1 | wc -l
> 243
> automaton@0ce59d338-1:~/cassandra-trunk/logs$ grep 'Compacted.*standard1' debug.log-test1 | grep ") 1 sstable" | wc -l
> 218
> {noformat}
> By looking at the code, it appears that there's a way to directly edit the level of a particular SSTable like the following:
> {code}
> sstable.descriptor.getMetadataSerializer().mutateLevel(sstable.descriptor, targetLevel);
> sstable.reloadSSTableMetadata();
> {code}
> Compared to what we have now (reading the whole single-SSTable from old level and writing out the same single-SSTable at the new level), the only difference I could think of by using this approach is that the new SSTable will have the same file name (sequence number) as the old one's, which could break some assumptions on some other part of the code. However, not having to go through the full read/write IO, and not having to bear the overhead of cleaning up the old file, creating the new file, creating more churns in heap and file buffer, it seems the benefits outweigh the inconvenience. So I'd argue this JIRA belongs to LHF and should be made available in 3.0.x as well.



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