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_r339368185
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File path:
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SequenceIdAccounting.java
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@@ -356,9 +356,36 @@ 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 and then cause data loss maybe.
+ // The solution here is a bit hack but fine. Just replace the
lowestUnflushedSeqId with
+ // maxFlushedSeqId + 1 if it is lesser. Durability.ASYNC_WAL means we do
not care data loss for
Review comment:
Should we +1 it and not just let it be the maxFlushedSeqID? The
maxFlushedSeqId comes from mvcc. +1'ing outside of mvcc management could have
an interesting side-effect not seen currently where next edit given out by mvcc
might clash w/ this +1'd record.
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