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https://issues.apache.org/jira/browse/CASSANDRA-14229?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16363636#comment-16363636
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Jeff Jirsa commented on CASSANDRA-14229:
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Sorta dove-tails in with CASSANDRA-8460 - similar purpose, different selection
criteria, but could potentially use some of the same concepts.
> Separate data drive for Index.db files
> --------------------------------------
>
> Key: CASSANDRA-14229
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14229
> Project: Cassandra
> Issue Type: New Feature
> Components: Local Write-Read Paths
> Reporter: Dan Kinder
> Priority: Minor
>
> For datasets with an active set of keys that well exceeds ram, it would be
> quite useful to be able to put certain sstable files (e.g. *-Index.db) on a
> separate, faster drive(s) than the data. E.g. put the indexes on SSD and data
> on HDD. Particularly valuable when keys are much smaller than values. Also as
> ram continues to get more expensive, users that currently optimize by having
> large key caches may not need to buy as much of it.
> Our use case is a large dataset like this one. Storing all the data on SSD is
> cost-prohibitive, and the reads are extremely random (effectively every key
> is in the active set), so we don't have enough ram to cache it. (I did try
> using a massive key cache, 64GB, and was seeing strange behavior anyway...
> irqbalancer process pegged the cpu and the whole thing way underperformed. An
> investigation for another day.)
> At the moment our only resolution is to buy enough HDD to handle 2 seeks per
> read, 1 for the index and 1 for the data. But having indexes on SSD would
> speed this way up, and practically require us to purchase a small number of
> SSDs and about 1/2 the number of HDD.
> One user suggested lvmcache, which could work. I'd like to hear if this will
> really work optimally and if lvmcache will really keep the right blocks on
> the faster volume, and how reliable it is at the task.
> Note: asked about this on the mailing list and it was suggested I create a
> JIRA.
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