[jira] Commented: (HBASE-2468) Improvements to prewarm META cache on clients

["Mingjie Lai (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-2468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12866758#action_12866758 ] 

Mingjie Lai commented on HBASE-2468:
------------------------------------

I'm looking at this issue right now, and want to provide a patch it as a warmup. 

My plan is to add a new HConnectionManager::locateRegionInMeta() which is dedicated for .META. table. It keeps the features of the original version (locate region for a table+row pair), in addition it puts all this table's region info to local cache by usig MetaScanner. 


> 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: 0.20.5
>
>
> 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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