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https://issues.apache.org/jira/browse/HBASE-8701?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13686149#comment-13686149
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Himanshu Vashishtha commented on HBASE-8701:
--------------------------------------------
Thanks for the patch, Ted and Jeffrey.
Few comments:
Once this is done, we have HFiles with positive and negative mvcc numbers?
{code}
- if (kv.getMemstoreTS() <= smallestReadPoint) {
+ if (kv.getMemstoreTS() > 0 && kv.getMemstoreTS() <= smallestReadPoint)
{
kv.setMemstoreTS(0);
}
{code}
Now, when do we set it back to 0?
Is it possible to have some doc on the scanner changes... Yeah, it needs
scanner level change.
Anyway, we were trying to fix the mttr, but now it looks we are changing code
path that is used by the core.. scan/get. Could it have some performance
implications?
bq. Chatting w/ Himanshu, he wondered if it is possible that a memstore get
flushed w/ a negative mvcc?
Yea, looks like there is no check for not flushing, etc while we are writing to
the region. What happens if we are done with the replay, make the region up for
read, but compaction has not kicked in yet.
Re: "bumping up the sequence number"
bq. + nextSeqid += SEQNUM_SAFETY_BUMPER;
Stack pointed out a case in 8741, where a region (with exceptionally high
sequenceId) is opened to a region server, and that regionserver dies
immediately after opening it. I think the bumping-up scheme will not be
sufficient.
> distributedLogReplay need to apply wal edits in the receiving order of those
> edits
> ----------------------------------------------------------------------------------
>
> Key: HBASE-8701
> URL: https://issues.apache.org/jira/browse/HBASE-8701
> Project: HBase
> Issue Type: Bug
> Components: MTTR
> Reporter: Jeffrey Zhong
> Assignee: Jeffrey Zhong
> Fix For: 0.98.0, 0.95.2
>
> Attachments: 8701-v3.txt, hbase-8701-v4.patch
>
>
> This issue happens in distributedLogReplay mode when recovering multiple puts
> of the same key + version(timestamp). After replay, the value is
> nondeterministic of the key
> h5. The original concern situation raised from [~eclark]:
> For all edits the rowkey is the same.
> There's a log with: [ A (ts = 0), B (ts = 0) ]
> Replay the first half of the log.
> A user puts in C (ts = 0)
> Memstore has to flush
> A new Hfile will be created with [ C, A ] and MaxSequenceId = C's seqid.
> Replay the rest of the Log.
> Flush
> The issue will happen in similar situation like Put(key, t=T) in WAL1 and
> Put(key,t=T) in WAL2
> h5. Below is the option(proposed by Ted) I'd like to use:
> a) During replay, we pass original wal sequence number of each edit to the
> receiving RS
> b) In receiving RS, we store negative original sequence number of wal edits
> into mvcc field of KVs of wal edits
> c) Add handling of negative MVCC in KVScannerComparator and KVComparator
> d) In receiving RS, write original sequence number into an optional field of
> wal file for chained RS failure situation
> e) When opening a region, we add a safety bumper(a large number) in order for
> the new sequence number of a newly opened region not to collide with old
> sequence numbers.
> In the future, when we stores sequence number along with KVs, we can adjust
> the above solution a little bit by avoiding to overload MVCC field.
> h5. The other alternative options are listed below for references:
> Option one
> a) disallow writes during recovery
> b) during replay, we pass original wal sequence ids
> c) hold flush till all wals of a recovering region are replayed. Memstore
> should hold because we only recover unflushed wal edits. For edits with same
> key + version, whichever with larger sequence Id wins.
> Option two
> a) During replay, we pass original wal sequence ids
> b) for each wal edit, we store each edit's original sequence id along with
> its key.
> c) during scanning, we use the original sequence id if it's present otherwise
> its store file sequence Id
> d) compaction can just leave put with max sequence id
> Please let me know if you have better ideas.
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