max seq id in flushed file can be larger than its correct value causing data
loss during recovery
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Key: HBASE-3481
URL: https://issues.apache.org/jira/browse/HBASE-3481
Project: HBase
Issue Type: Bug
Reporter: Kannan Muthukkaruppan
Priority: Critical
[While doing some cluster kill tests, I noticed some missing data after log
recovery. Upon investigating further, and pretty printing contents of HFiles
and recovered logs, this is my analysis of the situation/bug. Please confirm
the theory and pitch in with suggestions.]
When memstores are flushed, the max sequence id recorded in the HFile should
be the max sequence id of all KVs in the memstore. However, we seem to be
simply obtain the current sequence id from the HRegion, and stamp the HFile's
MAX_SEQ_ID with it.
>From HRegion.java:
{code}
sequenceId = (wal == null)? myseqid: wal.startCacheFlush();
{code}
where, startCacheFlush() is:
{code}
public long startCacheFlush() {
this.cacheFlushLock.lock();
return obtainSeqNum();
}
{code}
where, obtainSeqNum() is simply:
{code}
public long startCacheFlush() {
this.cacheFlushLock.lock();
return obtainSeqNum();
}
{code}
So let's say a memstore contains edits with sequence number 1..10.
Meanwhile, say more Puts come along, and are going through this flow (in
pseudo-code)
{code}
1. HLog.append();
1.1 obtainSeqNum()
1.2 writeToWAL()
2 updateMemStore()
{code}
So it is possible that the sequence number has already been incremented to say
15 (if there are 5 more outstanding puts)... but if their writeToWAL() is still
in progress. In this case, none of these edits (11..15) would have been written
to memstore yet.
At this point if a cache flush of the memstore happens, then we'll record its
MAX_SEQ_ID as 16 instead of 10 (because that's what obtainSeqNum() would return
as the next sequence number to use, right?).
Assume that the edits 11..15 eventually complete. And so HLogs do contain the
data for edits 11..15.
Now, at this point if the region server were to crash, and we run log recovery,
the splits all go through correctly, and a correct recovered.edits file is
generated with the edits 11..15.
Next, when the region is opened, the HRegion notes that one of the store file
says MAX_SEQ_ID is 16. So, when it replays the recovered.edits file, it skips
replaying edits 11..15. Or in other words, data loss.
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