[jira] Updated: (HBASE-1823) Ability for Scanners to bypass the block cache

["Jonathan Gray (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HBASE-1823?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Jonathan Gray updated HBASE-1823:
---------------------------------

    Attachment: HBASE-1823-v1.patch

Pretty simple change.  Adds a boolean flag, cacheBlocks, to Scan with a getter and setter.  That boolean is passed into the existing HFile.Reader.getScanner(boolean cacheBlocks) that was created to allow this functionality during compactions.

> Ability for Scanners to bypass the block cache
> ----------------------------------------------
>
>                 Key: HBASE-1823
>                 URL: https://issues.apache.org/jira/browse/HBASE-1823
>             Project: Hadoop HBase
>          Issue Type: New Feature
>          Components: client, regionserver
>    Affects Versions: 0.20.0
>            Reporter: Jonathan Gray
>            Assignee: Jonathan Gray
>             Fix For: 0.20.1, 0.21.0
>
>         Attachments: HBASE-1823-v1.patch
>
>
> There are a number of use cases where exposing the ability to not cache blocks during a scan would be valuable.  For example, running row counts.
> The LRU is scan-resistant, so it does provide some protection already, but even in that case all you prevent is the eviction of hot blocks.  The LRU still runs many evictions and the blocks are referenced for much longer periods of time, so this adds enormous stress to the GC.
> Compactions already do this.  This issue is about exposing that as a switch to the client-side Scan object (will also enable it for MR jobs then).

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