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From "Karthik Ranganathan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-3327) For increment workloads, retain memstores in memory after flushing them
Date Thu, 09 Dec 2010 19:40:00 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3327?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12969892#action_12969892
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Karthik Ranganathan commented on HBASE-3327:
--------------------------------------------

True - I mentioned HLog limit because we observed it because of that, but this would address
the underlying issue for any of the reasons to flush. Additionally, this also makes it resilient
in the face of compactions, which HLog compactions would not help with. 

HLog compactions would also be most effective for the ICV kind of workload (frequent updates
to existing data) right?


> 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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