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Message-ID: <31587741.145311292447412467.JavaMail.jira@thor>
Date: Wed, 15 Dec 2010 16:10:12 -0500 (EST)
From: "Jonathan Gray (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Subject: [jira] Commented: (HBASE-3327) For increment workloads, retain
 memstores in memory after flushing them
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    [ https://issues.apache.org/jira/browse/HBASE-3327?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12971837#action_12971837 ] 

Jonathan Gray commented on HBASE-3327:
--------------------------------------

I actually disagree that the biggest benefit is 1 memstore plus snapshot.  That would then cover flushes but not compactions.  As stated, flushing w/ cacheOnWrite would be virtually the same but consume 25% the memory.  So for this case, I don't see the clear benefit of retaining the snapshot vs. cacheOnWrite of the flushed file.

This change is significant and would require a good bit of modifications to the tracking of aggregate MemStore sizes and the rules around eviction when under global heap pressure.  I still do like this idea in general but not sure it's the best direction for effort to be spent right now.

> For increment workloads, retain memstores in memory after flushing them
> -----------------------------------------------------------------------
>
>                 Key: HBASE-3327
>                 URL: https://issues.apache.org/jira/browse/HBASE-3327
>             Project: HBase
>          Issue Type: Improvement
>          Components: regionserver
>            Reporter: Karthik Ranganathan
>
> This is an improvement based on our observation of what happens in an increment workload. The working set is typically small and is contained in the memstores. 
> 1. The reason the memstores get flushed is because the number of wal logs limit gets hit. 
> 2. This in turn triggers compactions, which evicts the block cache. 
> 3. Flushing of memstore and eviction of the block cache causes disk reads for increments coming in after this because the data is no longer in memory.
> We could solve this elegantly by retaining the memstores AFTER they are flushed into files. This would mean we can quickly populate the new memstore with the working set of data from memory itself without having to hit disk. We can throttle the number of such memstores we retain, or the memory allocated to it. In fact, allocating a percentage of the block cache to this would give us a huge boost.

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