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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-7011) log rolling and cache flushing should be able to proceed in parallel
Date Fri, 19 Oct 2012 05:00:04 GMT

    [ https://issues.apache.org/jira/browse/HBASE-7011?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13479612#comment-13479612

stack commented on HBASE-7011:

I like Todd's reasoning.  Not sure that the original intent.  It looks like we've been writing
special bytes on flush completion to the WAL since first check in (See 'HADOOP-1045.  Add
contrib/hbase, a BigTable-like online database.'  Look in HLog#completeCacheFlush).  It looks
like the intent was other than what Todd described going by comments at head of the class
and the described intent as never implemented.  Grep CACHEFLUSH.

Your recounting of how things works now sounds good.   You say the zombie RS 'should fail'
when the dir is renamed under it.  I haven't looked but if we don't already, sounds like we
should add a test that proves it.

Good on you Kannan.
> log rolling and cache flushing should be able to proceed in parallel
> --------------------------------------------------------------------
>                 Key: HBASE-7011
>                 URL: https://issues.apache.org/jira/browse/HBASE-7011
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Kannan Muthukkaruppan
>            Assignee: Kannan Muthukkaruppan
> Today, during a memstore flush (snapshot of memstore + flushing to disk), log rolling
cannot happen. This seems like a bad design, and an unnecessary restriction. 
> Possible reasons cited for this in code are:
> (i) maintenance of the lastSeqWritten map.
> (ii) writing a "completed-cache-flush" marker into the same log before the roll.
> It seems that we can implement a new design for (i) to avoid holding the lock for the
entire duration of the flush. And the motivation for (ii) is not even clear. We should reason
this out, and make sure we can relax the restriction. [See related discussion in HBASE-6980.]

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