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https://issues.apache.org/jira/browse/CASSANDRA-4937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13577878#comment-13577878
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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 fadvising.
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: 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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