[jira] [Resolved] (HBASE-9840) Large scans and BlockCache evictions problems

["Lars Hofhansl (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HBASE-9840?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

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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