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https://issues.apache.org/jira/browse/HBASE-2468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12870330#action_12870330
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Jonathan Gray commented on HBASE-2468:
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A single table could be made up of many thousands even hundreds of thousands of
regions? It could be significant. Even if 1000 regions it could add a huge
overhead to instantiating an HTable a user may only use for a single Get.
As for the look-ahead, I think it makes sense if there is a cached value that
gets evicted to do a look-ahead of an extra row so you have a good chance of
picking up the other side of a split. Beyond that, I don't think it's a good
idea to add a default overhead because this fetch is likely blocking a user
data request waiting for a region to open and refresh the location from meta.
I love these features and have wanted them for a while. Just don't think we
should change default HTable behavior significantly.
> Improvements to prewarm META cache on clients
> ---------------------------------------------
>
> Key: HBASE-2468
> URL: https://issues.apache.org/jira/browse/HBASE-2468
> Project: Hadoop HBase
> Issue Type: Improvement
> Components: client
> Reporter: Todd Lipcon
> Assignee: Mingjie Lai
> Fix For: 0.21.0
>
> Attachments: HBASE-2468-trunk.patch
>
>
> A couple different use cases cause storms of reads to META during startup.
> For example, a large MR job will cause each map task to hit meta since it
> starts with an empty cache.
> A couple possible improvements have been proposed:
> - MR jobs could ship a copy of META for the table in the DistributedCache
> - Clients could prewarm cache by doing a large scan of all the meta for the
> table instead of random reads for each miss
> - Each miss could fetch ahead some number of rows in META
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