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https://issues.apache.org/jira/browse/CASSANDRA-15452?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17844826#comment-17844826
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Jordan West commented on CASSANDRA-15452:
-----------------------------------------
I wrote a [prototype|https://github.com/jrwest/cassandra/tree/jwest/15452-5.0]
that uses a 256KB off-heap buffer for non-mmap'd, compressed data files only
and tested it with [~rustyrazorblade]. The test uses "big" format. We have yet
to validate with BTI format.
*To summarize our findings when running a single compaction:*
* *on NVMe: equal throughput with an approx 5x reduction in IOPS*
* *on EBS: 3x throughput with an approx 5x reduction in IOPS*
Attached are graphs of the iostat data and the raw iostat output from the EBS
tests.
*EBS (patched):*
!iostat-ebs-15452.png!
*EBS (5.0-HEAD):*
!iostat-ebs-head.png!
*NVMe (patched):*
!iostat-instance-15452.png!
*NVMe (5.0-HEAD):*
!iostat-instance-head.png!
[^iostat-5.0-head.output]
[^iostat-5.0-patched.output]
> 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, iostat-5.0-head.output,
> iostat-5.0-patched.output, iostat-ebs-15452.png, iostat-ebs-head.png,
> iostat-instance-15452.png, iostat-instance-head.png, 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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