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https://issues.apache.org/jira/browse/HBASE-11401?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14154270#comment-14154270
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Anoop Sam John commented on HBASE-11401:
----------------------------------------
I tried different ways to solve this. But only at this point we are sure abt
the seqNo being assigned to Cells
{code}
// ------------------------------------------------------------------
// STEP 8. Advance mvcc. This will make this put visible to scanners and
getters.
// ------------------------------------------------------------------
if (w != null) {
mvcc.completeMemstoreInsertWithSeqNum(w, walKey);
w = null;
}
{code}
What I tried to make sure to add the Cell only after the seq number is
assigned. But the above makes it not possible. Also just making the seqNo from
HLog first and then doing the insert and mvcc complete steps seems affecting
the performance.
Because of all these I would like to close this issue as 'Won't fix' as of now.
Over in HBASE-10713, I can solve the problem with a simple way. I do the cell
block flush (in - memory) in a thread and there I can wait for the previous
mvcc transactions to complete. This might delay the flush op a bit as it might
wait for Cells which are not part of this flush. But that is okey IMO.
> Late-binding sequenceid presumes a particular KeyValue mvcc format hampering
> experiment
> ---------------------------------------------------------------------------------------
>
> Key: HBASE-11401
> URL: https://issues.apache.org/jira/browse/HBASE-11401
> Project: HBase
> Issue Type: Bug
> Affects Versions: 0.99.0
> Reporter: Anoop Sam John
> Priority: Critical
> Fix For: 0.99.1
>
> Attachments: 11401.changing.order.txt, memstore.txt,
> nopatch.traces.svg, wpatch.traces.svg
>
>
> After HBASE-8763, we have combined KV mvcc and HLog seqNo. This is
> implemented in a tricky way now.
> In HRegion on write path, we first write to memstore and then write to HLog
> finally sync log. So at the time of write to memstore we dont know the WAL
> seqNo. To overcome this, we hold the ref to the KV objects just added to
> memstore and pass those also to write to wal call. Once the seqNo is
> obtained, we will reset the mvcc is those KVs with this seqNo. (While write
> to memstore we wrote kvs with a very high temp value for mvcc so that
> concurrent readers wont see them)
> This model works well with the DefaultMemstore. During the write there wont
> be any concurrent call to snapshot().
> But now we have memstore as a pluggable interface. The above model of late
> binding assumes that the memstore internal datastructure continue to refer to
> same java objects. This might not be true always. Like in HBASE-10713, in
> btw the kvs can be converted into a CellBlock. If we discontinue to refer to
> same KV java objects, we will fail in getting the seqNo assigned as kv mvcc.
> If we were doing write and sync to wal and then write to memstore, this would
> have get solved. But this model we changed (in 94 I believe) for better perf.
> Under HRegion level lock, we write to memstore and then to wal. Finally out
> of lock we do the the log sync. So we can not change it now
> I tried changing the order of ops within the lock (ie. write to log and then
> to memstore) so that we can get the seqNo when write to memstore. But because
> of the new HLog write model, we are not guarenteed to get the write to done
> immediately.
> One possible way can be add a new API in Log level, to get a next seqNo
> alone. Call this first and then using which write to memstore and then to wal
> (using this seqNo). Just a random thought. Not tried.
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