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From "Nicolas Liochon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10018) Change the location prefetch
Date Wed, 26 Feb 2014 23:21:24 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10018?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13913715#comment-13913715
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Nicolas Liochon commented on HBASE-10018:
-----------------------------------------

bq.  How much we prefetch when we do the reverse scan?
Just one. We could increase this, with a kind of heuristic to stay in 1 network packet, but
well..

bq. Should we get rid of HTable.getRowOrBefore() 
It's still used when you do a meta scan, even it's may be an historical case. I will double
check. In any case, we need to deprecate it first.

bq. but implement it via small reverse scan? 
We could do this, but I'm not sure that this feature is very useful outside of the .meta.
case, so...

bq. this will bring a regression
You mean a performance regression? It's complex, because we unlikely to be better in all cases,
and there is the usual difference between throughput and latency. And may be the reversed
scan is not efficient enough (I need to look at how it works when the scan is 'small', I'm
not sure this case is optimized actually).

> Change the location prefetch
> ----------------------------
>
>                 Key: HBASE-10018
>                 URL: https://issues.apache.org/jira/browse/HBASE-10018
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.98.0, 0.96.0
>            Reporter: Nicolas Liochon
>            Assignee: Nicolas Liochon
>             Fix For: 0.98.1, 0.99.0
>
>         Attachments: 10018.v1.patch, 10018.v2.patch
>
>
> Issues with prefetching are:
> - we do two calls to meta: one for the exact row, one for the prefetch 
> - it's done in a lock
> - we take the next 10 regions. Why 10, why the 10 next?
> - is it useful if the table has 100K regions?
> Options are:
> - just remove it
> - replace it with a reverse scan: this would save a call
>  



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