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From "Pavel Yaskevich (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4937) CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
Date Wed, 13 Feb 2013 21:22:13 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13577942#comment-13577942
] 

Pavel Yaskevich commented on CASSANDRA-4937:
--------------------------------------------

If that is optimization for big rows than it renders that feature useless if we have combination
of big/small rows mixed together because it doesn't take into account what actually is in
key cache. Also it doesn't take into account how big rows are distributed inside of the SSTable
so if they share a page with few small rows (which are in page cache) that we effectively
missing out on benefits that preheat feature gives us. 

I'm -1 committing that with that option because it's bad from operations perspective as it
could result in sudden degradation of latencies and would be impossible to understand without
actually knowing the code once 90% switch flips even if read subset of all data is still smaller
than page size.
                
> CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
> -------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-4937
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4937
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Pavel Yaskevich
>            Assignee: Pavel Yaskevich
>              Labels: core
>             Fix For: 1.2.2
>
>         Attachments: 4937-v3.txt, CASSANDRA-4937.patch, CASSANDRA-4937-trunk.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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