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https://issues.apache.org/jira/browse/CASSANDRA-15452?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17823391#comment-17823391
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Jon Haddad commented on CASSANDRA-15452:
----------------------------------------
I took another look at this. This lets us extract every read operation against
a single data file:
{noformat}
awk '$4 == "R" { print $0 }' everyfs.txt | grep '30-bti-Data.db' >
30-bti-data.txt{noformat}
If you glance at the end of the data, the last entry is this:
{noformat}
23:47:12 CompactionExec 44651 R 2699 12483 0.00
da-30-bti-Data.db{noformat}
The data file is only 15KB. But we're doing over 6 thousand reads
{noformat}
wc -l ../research/30-bti-data.txt
6420 ../research/30-bti-data.txt{noformat}
The 5th column is the number of bytes read. Summing this:
{noformat}
awk '{ sum += $5; } END {print sum}' ../research/30-bti-data.txt
25571844{noformat}
= 25MB
which is a lot to pull through the filesystem when in an optimal situation have
done a single 16KB read.
> 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
> 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.
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