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https://issues.apache.org/jira/browse/HBASE-7336?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14062356#comment-14062356
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Vladimir Rodionov commented on HBASE-7336:
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h3. Effect of compaction on large scan operations and vice verse
The compaction scanner and user scanner will compete for the same input stream
(DFSInputStream). This results in a sub optimal performance for both of them,
becuase *there is no guarantee that next call to read HFile block from the
"lucky" scanner will use the same streaming API and pre-cached data will still
be valid*. Yep? Both scanners, periodically, switch between stream/pread API
calls, hdfs cache can not be used (?), performance of both of them is defined
by positional read performance (which is low for scan mode operation).
Is this correct assessment?
> HFileBlock.readAtOffset does not work well with multiple threads
> ----------------------------------------------------------------
>
> Key: HBASE-7336
> URL: https://issues.apache.org/jira/browse/HBASE-7336
> Project: HBase
> Issue Type: Sub-task
> Components: Performance
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Priority: Critical
> Fix For: 0.94.4, 0.95.0
>
> Attachments: 7336-0.94.txt, 7336-0.96.txt
>
>
> HBase grinds to a halt when many threads scan along the same set of blocks
> and neither read short circuit is nor block caching is enabled for the dfs
> client ... disabling the block cache makes sense on very large scans.
> It turns out that synchronizing in istream in HFileBlock.readAtOffset is the
> culprit.
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