[HBASE-13922] Do not reset mvcc in compactions for mob-enabled column

• Type: Sub-task
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: hbase-11339
• Fix Version/s: hbase-11339, 2.0.0
• Component/s: mob
• Hadoop Flags: Reviewed
• Assignee: Jingcheng Du
• Reporter: Jingcheng Du
• Created: 17/Jun/15 07:34
• Updated: 17/Jun/22 05:58
• Resolved: 29/Jun/15 08:00

Description

In major compaction, the mvcc of cells ( whose mvcc<=readPt) are set to 0.
In some cases, this brings issues, for example the following scenario:

1. We have mob enabled cf, the threshold is 5 bytes.
2. Add a cell (r0,ts0,seqId=5,"mobValue0"), and flush it to a mob file.
3. Add another cell (r0,ts0,seqId=10,"new"), and flush the memstore, this is not a mob cell since it's value is smaller than 5 bytes.
4. Add the third cell (r1:ts1:seqId =15, "mobValue1"), and flush it to a mob file. Now we have two mob files.
5. Now run a major compaction in hfiles, we got two cells left (r0:ts0:seqId=0,"new") and (r1:ts1:seqId=0,'mobValue1").
6. Now run a mob major compaction, two mob files are merged into one. The update ref cell is bulk loaded back to hbase, they are (r0,ts0,seqId=5,"mobValue0") and (r1:ts1:seqId=0,"mobValue1").
7. Now open a scanner, the value of r0 is mobValue0 whereas the correct value new.

This issue is caused by the mvcc reset in compactions. We should disable it in compactions for mob-enabled columns.