saintstack commented on a change in pull request #762:
URL: https://github.com/apache/hbase/pull/762#discussion_r467612469
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File path:
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SequenceIdAccounting.java
##########
@@ -351,9 +351,36 @@ Long startCacheFlush(final byte[] encodedRegionName, final
Map<byte[], Long> fam
return lowestUnflushedInRegion;
}
- void completeCacheFlush(final byte[] encodedRegionName) {
+ void completeCacheFlush(byte[] encodedRegionName, long maxFlushedSeqId) {
+ // This is a simple hack to avoid maxFlushedSeqId go backwards.
+ // The system works fine normally, but if we make use of
Durability.ASYNC_WAL and we are going
+ // to flush all the stores, the maxFlushedSeqId will be next seq id of the
region, but we may
+ // still have some unsynced WAL entries in the ringbuffer after we call
startCacheFlush, and
+ // then it will be recorded as the lowestUnflushedSeqId by the above
update method, which is
+ // less than the current maxFlushedSeqId. And if next time we only flush
the family with this
+ // unusual lowestUnflushedSeqId, the maxFlushedSeqId will go backwards.
+ // This is an unexpected behavior so we should fix it, otherwise it may
cause unexpected
+ // behavior in other area.
+ // The solution here is a bit hack but fine. Just replace the
lowestUnflushedSeqId with
+ // maxFlushedSeqId + 1 if it is lesser. The meaning of maxFlushedSeqId is
that, all edits less
+ // than or equal to it have been flushed, i.e, persistent to HFile, so set
+ // lowestUnflushedSequenceId to maxFlushedSeqId + 1 will not cause data
loss.
+ // And technically, using +1 is fine here. If the maxFlushesSeqId is just
the flushOpSeqId, it
+ // means we have flushed all the stores so the seq id for actual data
should be at least plus 1.
+ // And if we do not flush all the stores, then the maxFlushedSeqId is
calculated by
+ // lowestUnflushedSeqId - 1, so here let's plus the 1 back.
+ Long wrappedSeqId = Long.valueOf(maxFlushedSeqId + 1);
Review comment:
Makes sense.
Your comment/explanation here is good. It is also where it should be, in the
class named SequenceIdAccounting. Though hard to follow, it should be possible
to read this one class to find how sequnceid accounting is done in the system
including tricks to keep the system working when the likes of ASYNC_WAL is
enabled.
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