cuijianwei created HBASE-10598:
----------------------------------
Summary: Written data can not be read out because
MemStore#timeRangeTracker might be updated concurrently
Key: HBASE-10598
URL: https://issues.apache.org/jira/browse/HBASE-10598
Project: HBase
Issue Type: Bug
Components: regionserver
Affects Versions: 0.94.16
Reporter: cuijianwei
In our test environment, we found that written data can't be read out
occasionally. After debugging, we find that maximumTimestamp/minimumTimestamp
of MemStore#timeRangeTracker might decrease/increase when
MemStore#timeRangeTracker is updated concurrently, which might make the
MemStore/StoreFile to be filtered incorrectly when reading data out. Let's see
how the concurrent updating of timeRangeTracker#maximumTimestamp cause this
problem.
Imagining there are two threads T1 and T2 putting two KeyValues kv1 and kv2.
kv1 and kv2 belong to the same Store(the same region), but contain different
rowkeys. Consequently, kv1 and kv2 could be updated concurrently. When we see
the implementation of HRegionServer#multi, kv1 and kv2 will be add to MemStore
by HRegion#doMiniBatchMutation#applyFamilyMapToMemstore. Then,
MemStore#internalAdd will be invoked and MemStore#timeRangeTracker will be
updated by TimeRangeTracker#includeTimestamp as follows:
{code}
private void includeTimestamp(final long timestamp) {
...
else if (maximumTimestamp < timestamp) {
maximumTimestamp = timestamp;
}
return;
}
{code}
Imagining the current maximumTimestamp is t0 before includeTimestamp invoked,
kv1.timestamp=t1, kv2.timestamp=t2, t1 and t2 are both set by user(then, user
knows the timestamp of kv1 and kv2), and t1 > t2. T1 and T2 will be executed
concurrently, therefore, the two threads might both find the current
maximumTimestamp is less than the timestamp of its kv. After that, T1 and T2
will both set maximumTimestamp to timestamp of its kv. If T1 set
maximumTimestamp before T2 doing that, the maximumTimestamp will be set to t2.
Then, before any new update with bigger timestamp has been applied to the
MemStore, if we try to read out kv1 by HTable#get and set the timestamp of
'Get' to t1, the StoreScanner will decide whether the MemStoreScanner(imagining
kv1 has not been flushed) should be selected as candidate scanner by the method
MemStoreScanner#shouldUseScanner. The MemStore won't be selected because
maximumTimestamp of the MemStore has been set to t2 (t2 < t1). Consequently,
the written kv1 can't be read out and kv1 is lost from user's perspective.
If the analysis of above is right, after maximumTimestamp of
MemStore#timeRangeTracker has been set to t2, user will experience data lass in
the following situations:
1. Before any new write with kv.timestamp > t1 has been add to the MemStore,
read request of kv1 with timestamp=t1 can not read kv1 out.
2. Before any new put with kv.timestamp > t1 has been add to the MemStore, if a
flush happened, the data of MemStore will be flushed to StoreFile with
StoreFile#maximumTimestamp set to t2. After that, any read request with
timestamp=t2 can not read kv1 before next compaction(the content of StoreFile
won't change and kv1.timestamp might also not be included even after
compaction).
The second situation is much more serious because the incorrect timeRange of
MemStore has been persisted to the file. And Similarly, the concurrent update
of TimeRangeTracker#minimumTimestamp may also cause this problem.
As a simple way to fix the problem, we could add synchronized to
TimeRangeTracker#includeTimestamp so that this method won't be invoked
concurrently.
--
This message was sent by Atlassian JIRA
(v6.1.5#6160)
