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https://issues.apache.org/jira/browse/CASSANDRA-15452?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17844835#comment-17844835
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Jon Haddad commented on CASSANDRA-15452:
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I should share some additional information and point out some things that might
not be immediately obvious in the graphs about the EBS test.
* The GP3 EBS volume was configured at 3K IOPs with 256MB throughput.
* All tests were run with an empty page cache by doing the following, then a
nodetool compact.
* Compaction was not throttled.
* Read ahead was set to 4KB.
* The EBS volume was, as expected, completely limited by IOPS, as it peaks
right at 3K, with read throughput on the drive around 18-20MB/s.
* The spikes in writes, presumably from fsync, cause the dip in read perf.
* The patched version had a significant reduction in IOPS usage, using only
~500 IOPS to achieve around 65MB/s, *more than a 3x* improvement in throughput
and a {*}6x improvement in IOPS usage{*}. In a production environment this
would lead to more predictable read latencies, as compaction I/O competes with
reads for disk resources.
* Multiple compaction threads should be able to achieve an even more
impressive improvement since we haven't yet reached the resource limit of the
underlying device.
* This patch should significantly improve node density when using EBS as
compaction is a significant bottleneck.
Some other general information:
* The patch does not yet take advantage of the fadvise call, which should help
a production environment significantly by preventing compaction from polluting
the page cache with pages that are about to be removed from the filesystem.
Overall I am very excited by the results we're seeing here and will follow up
with additional testing.
> 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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