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https://issues.apache.org/jira/browse/HBASE-9840?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Lars Hofhansl resolved HBASE-9840.
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Resolution: Later
> Large scans and BlockCache evictions problems
> ---------------------------------------------
>
> Key: HBASE-9840
> URL: https://issues.apache.org/jira/browse/HBASE-9840
> Project: HBase
> Issue Type: Bug
> Reporter: Lars Hofhansl
>
> I just ran into a scenario that baffled me first, but after some reflection
> makes sense. I ran a very large scan that filled up most of the block cache
> with my scan's data. I ran that scan a few times.
> That I ran a much smaller scan, and this scan will never get all its blocks
> cached if it does not fit entirely into the remaining BlockCache; regardless
> how I often I run it!
> The reason is that the blocks of the first large scan were all promoted.
> Since the 2nd scan did not fully fit into the cache all blocks are
> round-robin evicted as I rerun the scan. Thus those blocks will never get
> accessed more than once before they get evicted again.
> Since promoted blocks are not demoted the large scan's block will never be
> evicted unless we have another small enough scan/get that can promote its
> blocks.
> Not sure what the proper solution is, but it seems only a LRU cache that can
> expire blocks over time would solve this.
> Granted, this is a pretty special case.
> Edit: My usual spelling digressions.
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