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https://issues.apache.org/jira/browse/HBASE-26384?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17441111#comment-17441111
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Duo Zhang commented on HBASE-26384:
-----------------------------------
IIRC the master patch is just for adding the UT, we missed a line in the code
for branch-2. [~comnetwork] So maybe this is why we missed this line on
branch-2 as adding it back may cause data loss?
> Segment already flushed to hfile may still be remained in CompactingMemStore
> -----------------------------------------------------------------------------
>
> Key: HBASE-26384
> URL: https://issues.apache.org/jira/browse/HBASE-26384
> Project: HBase
> Issue Type: Bug
> Components: in-memory-compaction
> Affects Versions: 3.0.0-alpha-1, 2.4.8
> Reporter: chenglei
> Assignee: chenglei
> Priority: Major
> Labels: branch-2
> Fix For: 2.5.0, 3.0.0-alpha-2
>
>
> When {{CompactingMemStore}} prepares flushing,
> {{CompactingMemStore.snapshot}} invokes following
> {{CompactingMemStore.pushPipelineToSnapshot}} method to get {{Snapshot}},
> following line 570 and line 575 uses {{CompactionPipeline#version}} to track
> whether the Segments in {{CompactionPipeline#pipeline}} has changed since it
> gets {{VersionedSegmentsList}} in line 570 before emptying
> {{CompactionPipeline#pipeline}} in line 575.
> {code:java}
> 565 private void pushPipelineToSnapshot() {
> 566 int iterationsCnt = 0;
> 567 boolean done = false;
> 568 while (!done) {
> 569 iterationsCnt++;
> 570 VersionedSegmentsList segments =
> pipeline.getVersionedList();
> 571 pushToSnapshot(segments.getStoreSegments());
> 572 // swap can return false in case the pipeline was updated
> by ongoing compaction
> 573 // and the version increase, the chance of it happenning is
> very low
> 574 // In Swap: don't close segments (they are in snapshot now)
> and don't update the region size
> 575 done = pipeline.swap(segments, null, false, false);
> .......
> }
> {code}
> However, when {{CompactingMemStore#inMemoryCompaction}} executes
> {{CompactionPipeline#flattenOneSegment}}, it does not change
> {{CompactionPipeline#version}} , if there is an {{in memeory compaction}}
> which executes {{CompactingMemStore#flattenOneSegment}} between above line
> 570 and line 575, the {{CompactionPipeline#version}} not change, but the
> {{Segment}} in {{CompactionPipeline}} has changed. Because
> {{CompactionPipeline#version}} not change, {{pipeline.swap}} in above line
> 575 could think it is safe to invoke following
> {{CompactionPipeline#swapSuffix}} method to remove {{Segment}} in
> {{CompactionPipeline}} , but the {{Segment}} in {{CompactionPipeline}} has
> changed because of {{CompactingMemStore#flattenOneSegment}} , so the
> {{Segment}} not removed in following line 295 and still remaining in
> {{CompactionPipeline}}.
> {code:java}
> 293 private void swapSuffix(List<? extends Segment> suffix,
> ImmutableSegment segment,
> 294 boolean closeSegmentsInSuffix) {
> 295 pipeline.removeAll(suffix);
> 296 if(segment != null) pipeline.addLast(segment);
> ....
> {code}
> However {{CompactingMemStore.snapshot}} think it is successful and continues
> to flush the {{Segment}} got by {{CompactingMemStore.snapshot}} as normal,
> but the {{Segment}} with the same cells still be left in
> {{CompactingMemStore}}. Leaving {{Segment}} which already flushed in
> {{MemStore}} is dangerous: if a Major Compaction before the left {{Segment}}
> flushing, there may be data erroneous.
> My Fix in the PR is as following:
> # Increasing the {{CompactionPipeline#version}} in
> {{CompactingMemStore#flattenOneSegment}} .
> Branch-2 has this problem but master not， because the branch-2 patch for
> HBASE-18375 omitting this.
> # For {{CompactionPipeline#swapSuffix}} , explicitly checking that the
> {{Segment}} in {{suffix}} input parameter is same as the {{Segment}} in
> {{pipeline}} one by one from
> the last element to the first element of {{suffix}} ， I think explicitly
> throwing Exception is better than hiding error and causing subtle problem.
> I made separate PRs for master and branch-2 so the code for master and
> brach-2 could consistent and master could also has UTs for this problem.
> [PR#3777|https://github.com/apache/hbase/pull/3777] is for master and
> [PR#3779|https://github.com/apache/hbase/pull/3779] is for branch-2.The
> difference between them is patch for brach-2 including following code in
> {{CompactionPipeline.replaceAtIndex}} which not included in branch-2 patch
> for HBASE-18375:
> {code:java}
> // the version increment is indeed needed, because the swap uses
> removeAll() method of the
> // linked-list that compares the objects to find what to remove.
> // The flattening changes the segment object completely (creation
> pattern) and so
> // swap will not proceed correctly after concurrent flattening.
> version++;
> {code}
>
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