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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6311) Data error after majorCompaction caused by keeping MVCC for opened scanners
Date Fri, 06 Jul 2012 18:07:34 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13408192#comment-13408192
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Lars Hofhansl commented on HBASE-6311:
--------------------------------------

We're only retaining them as long as an OPEN scanner could potentially be affected by them
(kv.getMemstoreTS() > maxReadPointToTrackVersions), so should be OK. In fact we're doing
the same for all other KVs as well. In HBASE-5569 I attempted to extend that logic to deleted
rows, but I forgot the delete markers.

I also ran TestReplication locally. Passes. Going to commit.

Thanks for the patch Chunhui and the review/discussion Ram!
                
> Data error after majorCompaction caused by keeping MVCC for opened scanners
> ---------------------------------------------------------------------------
>
>                 Key: HBASE-6311
>                 URL: https://issues.apache.org/jira/browse/HBASE-6311
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>    Affects Versions: 0.94.0
>            Reporter: chunhui shen
>            Assignee: chunhui shen
>            Priority: Blocker
>             Fix For: 0.96.0, 0.94.1
>
>         Attachments: HBASE-6311-test.patch, HBASE-6311v1.patch, HBASE-6311v2.patch
>
>
> It is a big problem we found in 0.94, and you could reproduce the problem in Trunk using
the test case I uploaded.
> When we do compaction, we will use region.getSmallestReadPoint() to keep MVCC for opened
scanners;
> However,It will make data mistake after majorCompaction because we will skip delete type
KV but keep the put type kv in the compacted storefile.
> The following is the reason from code:
> In StoreFileScanner, enforceMVCC is false when compaction, so we could read the delete
type KV,
> However, we will skip this delete type KV in ScanQueryMatcher because following code
> {code}
> if (kv.isDelete())
> {
> ...
>  if (includeDeleteMarker
>             && kv.getMemstoreTS() <= maxReadPointToTrackVersions) {
>           System.out.println("add deletes,maxReadPointToTrackVersions="
>               + maxReadPointToTrackVersions);
>           this.deletes.add(bytes, offset, qualLength, timestamp, type);
>         }
> ...
> }
> {code}
> Here maxReadPointToTrackVersions = region.getSmallestReadPoint();
> and kv.getMemstoreTS() > maxReadPointToTrackVersions 
> So we won't add this to DeleteTracker.
> Why test case passed if remove the line MultiVersionConsistencyControl.setThreadReadPoint(smallestReadPoint);
> Because in the StoreFileScanner#skipKVsNewerThanReadpoint
> {code}
> if (cur.getMemstoreTS() <= readPoint) {
>       cur.setMemstoreTS(0);
>     }
> {code}
> So if we remove the line MultiVersionConsistencyControl.setThreadReadPoint(smallestReadPoint);
> Here readPoint is LONG.MAX_VALUE, we will set memStore ts as 0, so we will add it to
DeleteTracker in ScanQueryMatcher 
> Solution:
> We use smallestReadPoint of region when compaction to keep MVCC for OPENED scanner, So
we should retain delete type kv in output in the case(Already deleted KV is retained in output
to make old opened scanner could read this KV) even if it is a majorcompaction.

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