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https://issues.apache.org/jira/browse/HBASE-12411?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14217447#comment-14217447
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Lars Hofhansl commented on HBASE-12411:
---------------------------------------
Some numbers:
||scanners per region||pread forced||OS cache||time/s||
|1|yes|no|33|
|1|yes|yes|30|
|1|no|no|22|
|1|no|yes|20|
|5|no|yes|13|
|5|no|no|13.3|
|5|yes|yes|13.2|
|5|no|no|13.4|
|30|no|yes|12.9|
|30|yes|yes|12.8|
|30|yes|no|13.4|
|30|no|no|13.1|
Note: In the 1 scanner per region case I see no visible load on the data node
process. In all other cases, due to the involved p-reads I see considerable
load on the DN.
So it looks like p-read everywhere is not of much help.
> Optionally enable p-reads and private readers for compactions
> -------------------------------------------------------------
>
> Key: HBASE-12411
> URL: https://issues.apache.org/jira/browse/HBASE-12411
> Project: HBase
> Issue Type: Improvement
> Components: Performance
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Fix For: 2.0.0, 0.98.9, 0.99.2
>
> Attachments: 12411-v2.txt, 12411-v3.txt, 12411-v4.txt, 12411.txt
>
>
> In the light of HDFS-6735 we might want to consider refraining from seek +
> read completely and only perform preads.
> For example currently a compaction can lock out every other scanner over the
> file which the compaction is currently reading for compaction.
> At the very least we can introduce an option to avoid seek + read, so we can
> allow testing this in various scenarios.
> This will definitely be of great importance for projects like Phoenix which
> parallelize queries intra region (and hence readers will used concurrently by
> multiple scanner with high likelihood.)
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