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From "Hudson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8208) In some situations data is not replicated to slaves when deferredLogSync is enabled
Date Tue, 09 Apr 2013 07:34:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13626337#comment-13626337
] 

Hudson commented on HBASE-8208:
-------------------------------

Integrated in HBase-0.94-security #133 (See [https://builds.apache.org/job/HBase-0.94-security/133/])
    HBASE-8208 In some situations data is not replicated to slaves when deferredLogSync is
enabled (Jeffrey Zhong) (Revision 1465174)

     Result = SUCCESS
larsh : 
Files : 
* /hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java
* /hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/wal/HLog.java

                
> In some situations data is not replicated to slaves when deferredLogSync is enabled
> -----------------------------------------------------------------------------------
>
>                 Key: HBASE-8208
>                 URL: https://issues.apache.org/jira/browse/HBASE-8208
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.98.0, 0.94.6, 0.95.0
>            Reporter: Jeffrey Zhong
>            Assignee: Jeffrey Zhong
>             Fix For: 0.98.0, 0.94.7, 0.95.1
>
>         Attachments: hbase-8208-0.94.patch, hbase-8208.patch, hbase-8208-v1.patch, hbase-8208_v2.patch
>
>
> This is a subtle issue. When deferredLogSync is enabled, there are chances we could flush
data before syncing all HLog entries. Assuming we just flush the internal cache and the server
dies with some unsynced hlog entries. 
> Data is not lost at the source cluster while replication is based on WAL files and some
changes we flushed at the source won't be replicated the slave clusters. 
> Although enabling deferredLogSync with tolerances of data loss, it breaks the replication
assumption that whatever persisted in the source should be replicated to its slave clusters.

> In short, the slave cluster could end up with double losses: the data loss in the source
and some data stored in source cluster may not be replicated to slaves either.
> The fix of the issue isn't hard. Basically we can invoke sync during each flush when
replication is enabled for a region server. Since sync returns immediately when nothing to
sync so there should be no performance impact.
> Please let me know what you think!
> Thanks,
> -Jeffrey

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