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https://issues.apache.org/jira/browse/HBASE-5000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13167655#comment-13167655
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stack commented on HBASE-5000:
------------------------------
I added a bit of rough doc suggesting users not disable blockcache completely.
{code}
<section xml:id="disabling.blockcache"><title>Disabling
Blockcache</title>
<para>Do not turn off block cache (You'd do it by setting
<varname>hbase.block.cache.size</varname> to zero).
Currently we do not do well if you do this because the regionserver
will spend all its time loading hfile
indices over and over again. If your working set it such that block
cache does you no good, at least
size the block cache such that hfile indices will stay up in the
cache (you can get a rough idea
on the size you need by surveying regionserver UIs; you'll see index
block size accounted near the
top of the webpage).</para>
</section>
{code}
> 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
> Assignee: 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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