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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-1597) Prevent unnecessary caching of blocks during compactions
Date Fri, 03 Jul 2009 15:06:48 GMT

    [ https://issues.apache.org/jira/browse/HBASE-1597?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12727000#action_12727000
] 

Jonathan Gray commented on HBASE-1597:
--------------------------------------

Good point, Andrew.  Somehow I forgot about an issue I actually filed, HBASE-1521.

Really what I didn't like in my patch was how I re-instantiate the Reader as a CompactionReader,
since I can't cast up to it.  I did it in a way that just assigns all the internal variables
so there's not much real overhead besides the instantiation.  Seems like there could be a
prettier way but it works and performance impact should be negligible.

Thanks.

> Prevent unnecessary caching of blocks during compactions
> --------------------------------------------------------
>
>                 Key: HBASE-1597
>                 URL: https://issues.apache.org/jira/browse/HBASE-1597
>             Project: Hadoop HBase
>          Issue Type: Bug
>    Affects Versions: 0.20.0
>            Reporter: Jonathan Gray
>            Assignee: Jonathan Gray
>             Fix For: 0.20.0
>
>         Attachments: HBASE-1597-v1.patch
>
>
> When running any kind of compaction, we read every block of every storefile being compacted
into the block cache.
> We would like to reuse any already cached blocks, if available, but otherwise we should
not bog down the LRU with unnecessary blocks.
> This is not as bad as it was with the old LRU because the latest LRU implementation (HBASE-1460)
is scan-resistant.  This ensures that we are not causing massive eviction of the blocks that
are being read multiple times or from in-memory tables.  However, this does add to the GC-woes
of an import because each block gets further referenced, and for longer periods of time. 
There is also overhead in running the LRU evictions.

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