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From "HBase Review Board (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2468) Improvements to prewarm META cache on clients
Date Wed, 02 Jun 2010 08:14:34 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12874492#action_12874492
] 

HBase Review Board commented on HBASE-2468:
-------------------------------------------

Message from: "Mingjie Lai" <mjlai09@gmail.com>


bq.  On 2010-05-31 21:52:12, Todd Lipcon wrote:
bq.  > src/main/java/org/apache/hadoop/hbase/client/HTable.java, line 1075
bq.  > <http://review.hbase.org/r/98/diff/4/?file=872#file872line1075>
bq.  >
bq.  >     I feel like it would be cleaner to have the following methods be non-static,
so they don't need any arguments. that would reduce the number of functions by factor of two

That was what I wanted to do in the very beginning. I don't like so many functions either.
 

But there is one existing method: 
  public static boolean isTableEnabled(String tableName) ...
  public static boolean isTableEnabled(Configuration conf, String tableName) ...

It's one of the reason that I just want to keep the original coding style to be consistent.


In addition, I agree we can make isRegionCachePrefetchEnabled() etc. to be non-static. However,
enableRegionCachePrefetch(), and disableRegionCachePrefetch() must to be static since we want
to enable/disable a table without instantiate HTable. In another word, we cannot really dis/enable
cache prefetch for each HTable instance who have the same table name. While we can only enable/disable
based on table name. It is pretty similar as table enable/disable. 


bq.  On 2010-05-31 21:52:12, Todd Lipcon wrote:
bq.  > src/main/java/org/apache/hadoop/hbase/client/HConnectionManager.java, line 1608
bq.  > <http://review.hbase.org/r/98/diff/4/?file=871#file871line1608>
bq.  >
bq.  >     kill this function

I don't like it either. I will kill all ``is...Disabled()'' methods. 


bq.  On 2010-05-31 21:52:12, Todd Lipcon wrote:
bq.  > src/test/java/org/apache/hadoop/hbase/client/TestFromClientSide.java, line 3639
bq.  > <http://review.hbase.org/r/98/diff/4/?file=874#file874line3639>
bq.  >
bq.  >     I really like these unit tests, but I think you should use a row key for the
Get that isn't also a region start key, as it may expose different behavior. eg I think you
will end up with 11 cached regions instead of 10

As mentioned above, I reimplemented MetaScanner so it will start scanning Meta from the region
row where the given table row resides, instead of scanning from the next region row in Meta.
HTable.getRowOrBefore() is called in MetaScanner to achieve it, (however I'm not very sure
whether it is the most efficient way to do it or not). 

So for this unit test, no matter the passed row is a region start key or not, we will always
get 10 here. 


bq.  On 2010-05-31 21:52:12, Todd Lipcon wrote:
bq.  > src/main/java/org/apache/hadoop/hbase/client/HConnectionManager.java, lines 776-781
bq.  > <http://review.hbase.org/r/98/diff/4/?file=871#file871line776>
bq.  >
bq.  >     this block should go after the cache check below, right?

I reimplemented MetaScanner so it will start scanning Meta from the region where the given
user table row resides, instead of scanning from the next region row in Meta.

In this case, prefetchRegionCache() also fetches the queried table+row to the cache. Here
I put it before the cache check block, so it can load the result from cache directly. Otherwise
it may do an extra meta scan if cache prefetch is enabled. 

However if multiple threads accessing this block concurrently, this way will cause cache-prefetch
executed twice. But I think this case is pretty rare, so the penalty should be relatively
smaller. 

What do you think? 


- Mingjie


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> Improvements to prewarm META cache on clients
> ---------------------------------------------
>
>                 Key: HBASE-2468
>                 URL: https://issues.apache.org/jira/browse/HBASE-2468
>             Project: 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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