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From "Jeffrey Zhong (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-11099) Two situations where we could open a region with smaller sequence number
Date Thu, 01 May 2014 04:50:17 GMT

    [ https://issues.apache.org/jira/browse/HBASE-11099?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13986343#comment-13986343
] 

Jeffrey Zhong commented on HBASE-11099:
---------------------------------------

[~saint.ack@gmail.com] Is that possible that we assign the sequence number in AppendNoSync
call without late binding assignment because the order of changes in the "ring buffer" will
be preserved till real WAL sync?

> Two situations where we could open a region with smaller sequence number
> ------------------------------------------------------------------------
>
>                 Key: HBASE-11099
>                 URL: https://issues.apache.org/jira/browse/HBASE-11099
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>    Affects Versions: 0.99.0
>            Reporter: Jeffrey Zhong
>             Fix For: 0.99.0
>
>
> Recently I happened to run into code where we potentially could open region with smaller
sequence number:
> 1) Inside function: HRegion#internalFlushcache. This is due to we change the way WAL
Sync where we use late binding(assign sequence number right before wal sync).
> The flushSeqId may less than the change sequence number included in the flush which may
cause later region opening code to use a smaller than expected sequence number when we reopen
the region.
> {code}
> flushSeqId = this.sequenceId.incrementAndGet();
> ...
> mvcc.waitForRead(w);
> {code}
> 2) HRegion#replayRecoveredEdits where we have following code:
> {code}
> ...
>           if (coprocessorHost != null) {
>             status.setStatus("Running pre-WAL-restore hook in coprocessors");
>             if (coprocessorHost.preWALRestore(this.getRegionInfo(), key, val)) {
>               // if bypass this log entry, ignore it ...
>               continue;
>             }
>           }
> ...
>           currentEditSeqId = key.getLogSeqNum();
> {code} 
> If coprocessor skip some tail WALEdits, then the function will return smaller currentEditSeqId.
In the end, a region may also open with a smaller sequence number. This may cause data loss
because Master may record a larger flushed sequence Id and some WALEdits maybe skipped during
recovery if the region fail again.



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