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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 Wed, 13 Feb 2013 19:52:13 GMT


Pavel Yaskevich commented on CASSANDRA-4937:

bq. preheats row data if 90% of the rows in the sstable are under the page size that we're

I see the reason to do that if we have big rows (index promoted to the Index component so
we don't touch first page of a row) and we don't know where we would be hitting them but this
is why I don't think that 90% is a good idea 

  - we don't know distribution of those big rows so if we small row which was sharing page
with big row it's still good to preheat as we read on page basis.

  - if we still preheat first page that we didn't need it would actually be migrated by kernel
automatically with adoptive read-ahead for example.

  - if rows grow over time it would be a sadden change (flip-flop) in behavior/latencies.

  - even if 90% are bigger of the page size it's quiet possible that keys that we actually
migrated in the cache are in other 10%.
> CRAR improvements (object cache + CompressionMetadata chunk offset storage moved off-heap).
> -------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-4937
>                 URL:
>             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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