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From "Gregory Chanan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-7018) Fix and Improve TableDescriptor caching for bulk assignment
Date Mon, 22 Oct 2012 22:42:12 GMT

    [ https://issues.apache.org/jira/browse/HBASE-7018?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13481889#comment-13481889
] 

Gregory Chanan commented on HBASE-7018:
---------------------------------------

Test failures seem real.  I think that previously, we would load the latest version the HTableDescriptor
once on a call openRegion, but now, if the HTableDescriptor is already in the cache, we won't
load the latest version.  This seems like a real problem.

We could just store which tables we've re-loaded, and use the current "refresh" code, but
I think it's probably clearer to just blow that away and only keep the optimization about
not making multiple roundtrips to the NN.
                
> Fix and Improve TableDescriptor caching for bulk assignment
> -----------------------------------------------------------
>
>                 Key: HBASE-7018
>                 URL: https://issues.apache.org/jira/browse/HBASE-7018
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>            Reporter: Gregory Chanan
>            Assignee: Gregory Chanan
>             Fix For: 0.94.3, 0.96.0
>
>         Attachments: 7018-trunk.v2, HBASE-7018-94.patch, HBASE-7018-94-v2.patch, HBASE-7018-trunk.patch
>
>
> HBASE-6214 backported HBASE-5998 (Bulk assignment: regionserver optimization by using
a temporary cache for table descriptors when receiving an open regions request), but it's
buggy on 0.94 (0.96 appears correct):
> {code}
>     HTableDescriptor htd = null;
>     if (htds == null) {
>       htd = this.tableDescriptors.get(region.getTableName());
>     } else {
>       htd = htds.get(region.getTableNameAsString());
>       if (htd == null) {
>         htd = this.tableDescriptors.get(region.getTableName());
>         htds.put(region.getRegionNameAsString(), htd);
>       }
>     }
> {code}
> i.e. we get the tableName from the map but write the regionName.
> Even fixing this, it looks like there are areas for improvement:
> 1) FSTableDescriptors already has a cache (though it goes to the NameNode each time through
to check we have the latest copy.  May as well combine these two caches, might be a performance
win as well since we don't need to write to multiple caches.
> 2) FSTableDescriptors makes two RPCs to the NameNode when it encounters a new table.
 So the total number of RPCs necessary for a bulk assign (without caching is):
> #regions + #tables
> (with caching):
> min(#regions,#tables) + #tables = #tables + #tables = 2 * #tables
> We can make this only one RPC, yielding:
> #tables
> Probably not a big deal for most users, but in a multi-tenant situation where the number
of regions being bulk assigned approaches the number of tables being bulk assigned, this could
be a nice performance win.
> Benchmarks coming.

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