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https://issues.apache.org/jira/browse/CASSANDRA-8460?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16414547#comment-16414547
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Ben Slater commented on CASSANDRA-8460:
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Hi John
I've been setting the requirements from our (Instaclustr) point of view for
Lerh here so I thought I'd weigh in on why I'd rather see a Cassandra based
solution than LVM.
The requirement we're looking to target, as per the original JIRA, is people
who have data that is hot for a short period but then they need to keep around
for a long time with infrequent access (ie well defined rules on hot vs cold,
not deciding what is hot based on what was recently read).
Typically when I've seen this requirement people want:
1) The best possible performance for the hot data
2) Lowest cost of storage for the cold data
It seems to me that with LVM we're a not doing the best we could in terms of
either of these.
For performance, there is the write-through slow down you mentioned, depending
on where you draw the line on moving to slow disk vs the final TWCS compaction
you might have compactions pushing data you want to be quick out of cache and
if you used EBS for both the hot disk and the slow disk you are increasing
usage of the EBS bandwidth to copy to and from cache (although using local SSD
as the cache negates this last one).
In terms of cost, with LVM the fast disk is purely being used as cache rather
than a primary store so you are having to duplicate that amount of data storage
- whether that is significant probably depends on your desired ratio of fast to
slow disk and how cost sensitive you are.
Whether this downsides are worth the extra complexity is of course a matter of
judgement rather than facts so happy to go with the community consensus here
but thought I'd put in my POV.
Cheers
Ben
> 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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