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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6065) Log for flush would append a non-sequential edit in the hlog, leading to possible data loss
Date Tue, 29 May 2012 09:30:24 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6065?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13284692#comment-13284692
] 

Lars Hofhansl commented on HBASE-6065:
--------------------------------------

Lastly, since this can lose data, I think this warrants a 0.94.1 release soon.
                
> Log for flush would append a non-sequential edit in the hlog, leading to possible data
loss
> -------------------------------------------------------------------------------------------
>
>                 Key: HBASE-6065
>                 URL: https://issues.apache.org/jira/browse/HBASE-6065
>             Project: HBase
>          Issue Type: Bug
>          Components: wal
>            Reporter: chunhui shen
>            Assignee: chunhui shen
>            Priority: Critical
>             Fix For: 0.96.0, 0.94.1
>
>         Attachments: HBASE-6065.patch, HBASE-6065v2.patch
>
>
> After completing flush region, we will append a log edit in the hlog file through HLog#completeCacheFlush.
> {code}
> public void completeCacheFlush(final byte [] encodedRegionName,
>       final byte [] tableName, final long logSeqId, final boolean isMetaRegion)
> {
> ...
> HLogKey key = makeKey(encodedRegionName, tableName, logSeqId,
>             System.currentTimeMillis(), HConstants.DEFAULT_CLUSTER_ID);
> ...
> }
> {code}
> when we make the hlog key, we use the seqId from the parameter, and it is generated by
HLog#startCacheFlush,
> Here, we may append a lower seq id edit than the last edit in the hlog file.
> If it is the last edit log in the file, it may cause data loss.
> because 
> {code}
> HRegion#replayRecoveredEditsIfAny{
> ...
> maxSeqId = Math.abs(Long.parseLong(fileName));
>       if (maxSeqId <= minSeqId) {
>         String msg = "Maximum sequenceid for this log is " + maxSeqId
>             + " and minimum sequenceid for the region is " + minSeqId
>             + ", skipped the whole file, path=" + edits;
>         LOG.debug(msg);
>         continue;
>       }
> ...
> }
> {code}
> We may skip the splitted log file, because we use the lase edit's seq id as its file
name, and consider this seqId as the max seq id in this log file.

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