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From "Jeffrey Zhong (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-8208) Data could not be replicated to slaves when deferredLogSync is enabled
Date Thu, 28 Mar 2013 00:25:15 GMT
Jeffrey Zhong created HBASE-8208:

             Summary: Data could not be 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: 0.94.6, 0.95.0, 0.98.0
            Reporter: Jeffrey Zhong

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!


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