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From "Pavel Yaskevich (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-4937) CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
Date Sat, 10 Nov 2012 00:39:14 GMT


Pavel Yaskevich commented on CASSANDRA-4937:

bq. Still feels like a hack to me... Will think about alternatives.

You are welcome to think how to do so without involving more then one file, I have exploited
lhf possibility :)

bq. But isn't that the point, that we do know that compaction only* does sequential i/o?

Even so, we will need to expose RAR method to skip cache to SSTable iterators so they can
hint RAR to hit OS to remove cache after they are done with the row for example, which, for
example, would generate too many system calls if you have a small rows (or try to somehow
batch them together which would require coordination between sstable iterators) as well we
would have to teach other components, which want to skip cache, how to do so (e.g. streaming).
That individual approach creates nothing but complexing because reads would still be suffering
even if we change granularity of hints.

> CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
> -------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-4937
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>    Affects Versions: 1.1.6
>            Reporter: Pavel Yaskevich
>            Assignee: Pavel Yaskevich
>            Priority: Minor
>             Fix For: 1.1.7
>         Attachments: CASSANDRA-4937.patch
> After good amount of testing on one of the clusters it was found that in order to improve
read latency we need to minimize allocation rate that compression involves, that minimizes
GC (as well as heap usage) and substantially decreases latency on read heavy workloads. 
> I have also discovered that RAR skip cache harms performance in situation when reads
are done in parallel with compaction working with relatively big SSTable files (few GB and
more). The attached patch removes possibility to skip cache from compressed files (I can also
add changes to RAR to remove skip cache functionality as a separate patch). 

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