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https://issues.apache.org/jira/browse/CASSANDRA-8460?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16414820#comment-16414820
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Jon Haddad commented on CASSANDRA-8460:
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I'm not sure what else to tell you, [~slater_ben]. You just described a
perfect use case for the Linux page cache, TWCS, and dm-cache. I understand
what you're trying to say - that somehow dm-cache won't cache the right data,
and Cassandra will somehow do a better job than the kernel at understanding the
data we need to keep hot, but so far my experience leads me to disagree with
you that there would be an issue.
For data that's recently been compacted & read, that data is going to be in
your page cache. For data that's been recently "major compacted" in the
previous TWCS window, that will either be in page cache or the dm-cache. After
that, the data is just sitting around, so the access patterns will keep it
either in cache or out, depending on when it's accessed.
Ultimately what matters is reducing the hit miss on your disk to a minimum.
You do that by keeping frequently accessed data in the cache. Using a time
element (recent) without factoring in hot spots will actually get your a
_worse_ cache hit ratio, which will put more pressure on the slow disks,
driving up seeks, making it harder to meet the SLA.
> Make it possible to move non-compacting sstables to slow/big storage in DTCS
> ----------------------------------------------------------------------------
>
> Key: CASSANDRA-8460
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8460
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Marcus Eriksson
> Assignee: Lerh Chuan Low
> Priority: Major
> Labels: doc-impacting, dtcs
> Fix For: 4.x
>
>
> It would be nice if we could configure DTCS to have a set of extra data
> directories where we move the sstables once they are older than
> max_sstable_age_days.
> This would enable users to have a quick, small SSD for hot, new data, and big
> spinning disks for data that is rarely read and never compacted.
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