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https://issues.apache.org/jira/browse/CASSANDRA-15452?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17910867#comment-17910867
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Jon Haddad commented on CASSANDRA-15452:
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Stopping the load test, busting the page cache, and running upgradesstables -a
to force a bunch of compaction activity, here's what I'm seeing. cassandra2,
(10.0.2.171) has roughly twice the read throughput as the other 2 nodes, while
using 1/3 of the IOPS:
!image-2025-01-07-16-56-12-853.png|width=598,height=146!
!image-2025-01-07-16-57-29-134.png|width=613,height=191!
> Improve disk access patterns during compaction and range reads
> --------------------------------------------------------------
>
> Key: CASSANDRA-15452
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15452
> Project: Apache Cassandra
> Issue Type: Improvement
> Components: Legacy/Local Write-Read Paths, Local/Compaction
> Reporter: Jon Haddad
> Assignee: Jordan West
> Priority: Normal
> Fix For: 4.1.x, 5.0.x, 5.x
>
> Attachments: everyfs.txt, image-2024-11-22-16-17-23-194.png,
> image-2025-01-07-16-04-23-909.png, image-2025-01-07-16-56-12-853.png,
> image-2025-01-07-16-57-29-134.png, 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,
> screenshot-1.png, screenshot-2.png, screenshot-3.png, screenshot-4.png,
> screenshot-5.png, screenshot-6.png, sequential.fio, throughput-1.png,
> throughput.png
>
> Time Spent: 4h 10m
> Remaining Estimate: 0h
>
> 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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