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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6065) Log for flush would append a non-sequential edit in the hlog, may cause data loss
Date Tue, 22 May 2012 04:45:41 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6065?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13280717#comment-13280717
] 

ramkrishna.s.vasudevan commented on HBASE-6065:
-----------------------------------------------

@Chunhui
What type of dataloass you see here? is it the edit with HBASE::CACHEFLUSH that gets missed
here?
Ideally by design that edit is actually needed to show up to what point the flush has been
done and the same is added as an entry in HLog.
Even while recovering we tend to skip this entry.
{code}
           // Check this edit is for me. Also, guard against writing the special
            // METACOLUMN info such as HBASE::CACHEFLUSH entries
            if (kv.matchingFamily(HLog.METAFAMILY) ||
                !Bytes.equals(key.getEncodedRegionName(), this.regionInfo.getEncodedNameAsBytes()))
{
              skippedEdits++;
              continue;
                }
{code}
Did you find any other type of dataloss which i am not able to foresee here? Correct me if
am wrong.
                
> Log for flush would append a non-sequential edit in the hlog, may cause 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
>         Attachments: HBASE-6065.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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