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https://issues.apache.org/jira/browse/HBASE-2180?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12831193#action_12831193
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stack commented on HBASE-2180:
------------------------------
Really commit to TRUNK.
> Bad random read performance from synchronizing hfile.fddatainputstream
> ----------------------------------------------------------------------
>
> Key: HBASE-2180
> URL: https://issues.apache.org/jira/browse/HBASE-2180
> Project: Hadoop HBase
> Issue Type: Bug
> Reporter: ryan rawson
> Assignee: stack
> Fix For: 0.20.4, 0.21.0
>
> Attachments: 2180-v2.patch, 2180.patch
>
>
> deep in the HFile read path, there is this code:
> synchronized (in) {
> in.seek(pos);
> ret = in.read(b, off, n);
> }
> this makes it so that only 1 read per file per thread is active. this
> prevents the OS and hardware from being able to do IO scheduling by
> optimizing lots of concurrent reads.
> We need to either use a reentrant API (pread may be partially reentrant
> according to Todd) or use multiple stream objects, 1 per scanner/thread.
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