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https://issues.apache.org/jira/browse/CASSANDRA-8460?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16414793#comment-16414793
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Ben Slater commented on CASSANDRA-8460:
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Fair enough - the example was a bit of an oversimplification even for how I
would have guessed it work. Having read up a bit
([https://www.redhat.com/en/blog/improving-read-performance-dm-cache)] and
([https://www.kernel.org/doc/Documentation/device-mapper/cache-policies.txt)] I
suspect we've actually got a bit of a different model of the use cases we are
both imagining (and I haven't done a great job of describing what I have in
mind).
Consider you're building an IOT application that collects sensor data and has
some kind of UI for displaying readings. You want to be able to provide an
experience for your users where accessing today's data (the most common use) is
snappy while still providing the ability to go back in time a year but as it's
not common it's fine for access to that data to be slower.
In this scenario the recent data isn't "hot" in the sense that it is accessed
many times (I'm not sure there is a well defined term for what it is - maybe
"high priority" is better?) so it's hard for a caching algorithm (like smq)
based on frequency of access to work effectively (in fact the first access is
the one you want to be fast).
Does that make more sense as to where I'm coming from?
> 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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