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https://issues.apache.org/jira/browse/HBASE-5000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13167360#comment-13167360
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Lars Hofhansl edited comment on HBASE-5000 at 12/12/11 5:40 AM:
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+1 on documenting. Could state there that block caching can be disabled on a
per scan bases, and in fact should be disabled if the data scanned is known (or
likely to) not to fit in the cache.
Generally shouldn't we aim for loading the same block only once per JVM if
requested by multiple threads at the same time? Could do some kind of reference
counting to keep track of other requesters, or register a call back when the
block is loaded...
was (Author: lhofhansl):
+1 on documenting. Could state there that block caching can be disabled on
a per scan bases, and in fact should be disabled if the data scanned is known
(or likely to) not to fit in the cache.
Generally shouldn't aim for loading the same block only once per JVM if
requested by multiple threads at the same time? Could do some kind of reference
counting to keep track of other requesters, or register a call back when the
block is loaded...
> 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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