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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2468) Improvements to prewarm META cache on clients
Date Sat, 22 May 2010 00:06:17 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12870235#action_12870235

Andrew Purtell commented on HBASE-2468:

bq. I'm unsure if I like the approach of aggressively pre-warming though you don't ask for
it (prefetch on HTable instantiation, and auto look-ahead in locateRegionInMeta

This is what this issue is asking for as I read it. 

bq. I think I would prefer adding an additional constructor w/ a warm boolean to determine
whether META should be warmed for that table or not.

That makes sense to me as long as the boolean defaults to _true_. 

bq. I don't quite understand the prefetching in locateRegionInMeta(). If you want to pre-cache
meta, you should warm the HTable on construction. Is this to try to update neighboring regions
in case they got stale? 

If the cache misses if due to a split, scanning forward a couple of entries has a reasonable
chance of picking up related changes. Should be less than 10. 1? 2? 

> 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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