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Date: Tue, 24 Jun 2014 01:36:25 +0000 (UTC)
From: "Enis Soztutar (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: <JIRA.12723060.1403496982865.33734.1403573785683@arcas>
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Subject: [jira] [Commented] (HBASE-11401) Issue with seqNo binding for KV
 mvcc
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    [ https://issues.apache.org/jira/browse/HBASE-11401?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14041577#comment-14041577 ] 

Enis Soztutar commented on HBASE-11401:
---------------------------------------

bq. If we were doing write and sync to wal and then write to memstore, this would have get solved
My initial poc patch at HBASE-8763 actually reordered the write path so that it was appendNoSync + write to memstore + sync(). But this was before the late binding from the disruptor which actually does the ordering and seqNum assignment. So it should be appendNoSync() + sync() + memstore order I guess. 

> Issue with seqNo binding for KV mvcc
> ------------------------------------
>
>                 Key: HBASE-11401
>                 URL: https://issues.apache.org/jira/browse/HBASE-11401
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.99.0
>            Reporter: Anoop Sam John
>            Priority: Critical
>             Fix For: 0.99.0
>
>
> After HBASE-8763, we have combined KV mvcc and HLog seqNo. This is implemented in a tricky way now.
> In HRegion on write path, we first write to memstore and then write to HLog finally sync log. So at the time of write to memstore we dont know the WAL seqNo.  To overcome this, we hold the ref to the KV objects just added to memstore and pass those also to write to wal call. Once the seqNo is obtained, we will reset the mvcc is those KVs with this seqNo.  (While write to memstore we wrote kvs with a very high temp value for mvcc so that concurrent readers wont see them)
> This model works well with the DefaultMemstore.  During the write there wont be any concurrent call to snapshot(). 
> But now we have memstore as a pluggable interface. The above model of late binding assumes that the memstore internal datastructure continue to refer to same java objects. This might not be true always.  Like in HBASE-10713, in btw the kvs can be converted into a CellBlock. If we discontinue to refer to same KV java objects, we will fail in getting the seqNo assigned as kv mvcc.
> If we were doing write and sync to wal and then write to memstore, this would have get solved. But this model we changed (in 94 I believe) for better perf. Under HRegion level lock, we write to memstore and then to wal. Finally out of lock we do the the log sync.  So we can not change it now
> I tried changing the order of ops within the lock (ie. write to log and then to memstore) so that we can get the seqNo when write to memstore. But because of the new HLog write model, we are not guarenteed to get the write to done immediately. 
> One possible way can be add a new API in Log level, to get a next seqNo alone. Call this first and then using which write to memstore and then to wal (using this seqNo).  Just a random thought. Not tried.


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