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Date: Mon, 15 Mar 2010 21:35:27 +0000 (UTC)
From: "Kannan Muthukkaruppan (JIRA)" <jira@apache.org>
To: hbase-issues@hadoop.apache.org
Subject: [jira] Commented: (HBASE-2283) row level atomicity
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    [ https://issues.apache.org/jira/browse/HBASE-2283?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12845540#action_12845540 ] 

Kannan Muthukkaruppan commented on HBASE-2283:
----------------------------------------------

scatch my comment (@15/Mar/10 07:14 PM). I was misreading my debug output. The ongoing scan is indeed unaffected by the delete.  There is some other issue with this test + my patch that I am still trying to nail down.

> row level atomicity 
> --------------------
>
>                 Key: HBASE-2283
>                 URL: https://issues.apache.org/jira/browse/HBASE-2283
>             Project: Hadoop HBase
>          Issue Type: Bug
>            Reporter: Kannan Muthukkaruppan
>            Priority: Blocker
>             Fix For: 0.20.4, 0.21.0
>
>         Attachments: rowLevelAtomicity_2283_v1.patch
>
>
> The flow during a HRegionServer.put() seems to be the following. [For now, let's just consider single row Put containing edits to multiple column families/columns.]
> HRegionServer.put() does a:
>         HRegion.put();
>        syncWal()  (the HDFS sync call).  /* this is assuming we have HDFS-200 */
> HRegion.put() does a:
>   for each column family 
>   {
>       HLog.append(all edits to the colum family);
>       write all edits to Memstore;
>   }
> HLog.append() does a :
>   foreach edit in a single column family {
>     doWrite()
>   }
> doWrite() does a:
>    this.writer.append().
> There seems to be two related issues here that could result in inconsistencies.
> Issue #1: A put() does a bunch of HLog.append() calls. These in turn do a bunch of "write" calls on the underlying DFS stream.  If we crash after having written out some append's to DFS, recovery will run and apply a partial transaction to memstore.  
> Issue #2: The updates to memstore  should happen after the sync rather than before. Otherwise, there is the danger that the write to DFS (sync) fails for some reason & we return an error to the client, but we have already taken edits to the memstore. So subsequent reads will serve uncommitted data.

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