saintstack commented on a change in pull request #762: HBASE-23157 WAL
unflushed seqId tracking may wrong when Durability.AS…
URL: https://github.com/apache/hbase/pull/762#discussion_r362702382
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File path:
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SequenceIdAccounting.java
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@@ -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:
The +1 makes me wary. Just add these edits w/ maxFlushedSeqId? Wouldn't
that be safer.
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