[jira] [Commented] (HBASE-5000) Speed up simultaneous reads of a block when block caching is turned off

["stack (Commented) (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-5000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13166608#comment-13166608 ] 

stack commented on HBASE-5000:
------------------------------

I agree this priority minor.  We should probably add to the book that currently disabling block cache completed is not recommended.
                
> Speed up simultaneous reads of a block when block caching is turned off
> -----------------------------------------------------------------------
>
>                 Key: HBASE-5000
>                 URL: https://issues.apache.org/jira/browse/HBASE-5000
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Mikhail Bautin
>            Priority: Minor
>
> With block caching, when one client starts reading a block and another one comes around asking for the same block, the second client waits for the first one to finish reading and returns the block from cache. This is achieved by locking on the block offset using IdLock, a "sparse lock" primitive allowing to lock on arbitrary long numbers. However, in case there is no block caching, there is no reason to wait for other clients that are reading the same block. One challenge optimizing this that we don't necessary have accurate information about whether other HFile API clients interested in the block would cache it.
> Setting priority as minor, as it is very unusual to turn off block caching.

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