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Posted to commits@cassandra.apache.org by "Jon Haddad (Jira)" <ji...@apache.org> on 2024/05/01 22:04:00 UTC
[jira] [Commented] (CASSANDRA-15452) Improve disk access patterns during compaction and streaming
[ https://issues.apache.org/jira/browse/CASSANDRA-15452?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17842790#comment-17842790 ]
Jon Haddad commented on CASSANDRA-15452:
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I was able to verify we're double reading, and I've created CASSANDRA-19607 to document it. Fixing this patch may end up fixing CASSANDRA-19607 as well.
> Improve disk access patterns during compaction and streaming
> ------------------------------------------------------------
>
> Key: CASSANDRA-15452
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15452
> Project: Cassandra
> Issue Type: Improvement
> Components: Legacy/Local Write-Read Paths, Local/Compaction
> Reporter: Jon Haddad
> Assignee: Jordan West
> Priority: Normal
> Attachments: everyfs.txt, results.txt, sequential.fio
>
>
> On read heavy workloads Cassandra performs much better when using a low read ahead setting. In my tests I've seen an 5x improvement in throughput and more than a 50% reduction in latency. However, I've also observed that it can have a negative impact on compaction and streaming throughput. It especially negatively impacts cloud environments where small reads incur high costs in IOPS due to tiny requests.
> # We should investigate using POSIX_FADV_DONTNEED on files we're compacting to see if we can improve performance and reduce page faults.
> # This should be combined with an internal read ahead style buffer that Cassandra manages, similar to a BufferedInputStream but with our own machinery. This buffer should read fairly large blocks of data off disk at at time. EBS, for example, allows 1 IOP to be up to 256KB. A considerable amount of time is spent in blocking I/O during compaction and streaming. Reducing the frequency we read from disk should speed up all sequential I/O operations.
> # We can reduce system calls by buffering writes as well, but I think it will have less of an impact than the reads
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