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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8763) [BRAINSTORM] Combine MVCC and SeqId
Date Tue, 27 May 2014 21:38:04 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14010314#comment-14010314
] 

stack commented on HBASE-8763:
------------------------------

Patch looks great.

On commit, remove this comment.  Doesn't seem appropriate to the statement that follows:

+        // Record the mvcc for all transactions in progress.

You need to do this?

-
+      mvccNum.setValue(this.sequenceId.incrementAndGet());

I'm trying to keep it so all sequenceid increments happen inside HLog only.

You do it in a few places.  Does it have to increment?  Could you just get current?  Would
that work?

This should almost be a method because it happens more than a few times:

+          if(walKey == null){
+            // Append a faked WALEdit in order for SKIP_WAL updates to get mvccNum assigned
+            walKey = this.appendNoSyncNoAppend(this.log, mvccNum);
+          }

Rename SequenceNumberAssignor as SequenceNumber and method as getSequenceNumber?

If no one else reviews in next few days, I'll give it another go (having trouble concentrating
on this because I've looked at a few versions of this patch....).  Overall I think this is
excellent.

Maybe we should just commit and then tune up in new issues?

Not sure we can get our speedup back looking at this patch at the mo.

> [BRAINSTORM] Combine MVCC and SeqId
> -----------------------------------
>
>                 Key: HBASE-8763
>                 URL: https://issues.apache.org/jira/browse/HBASE-8763
>             Project: HBase
>          Issue Type: Improvement
>          Components: regionserver
>            Reporter: Enis Soztutar
>            Assignee: Jeffrey Zhong
>            Priority: Critical
>         Attachments: HBase MVCC & LogSeqId Combined.pdf, hbase-8736-poc.patch, hbase-8763-poc-v1.patch,
hbase-8763-v1.patch, hbase-8763-v2.patch, hbase-8763-v3.patch, hbase-8763-v4.patch, hbase-8763_wip1.patch
>
>
> HBASE-8701 and a lot of recent issues include good discussions about mvcc + seqId semantics.
It seems that having mvcc and the seqId complicates the comparator semantics a lot in regards
to flush + WAL replay + compactions + delete markers and out of order puts. 
> Thinking more about it I don't think we need a MVCC write number which is different than
the seqId. We can keep the MVCC semantics, read point and smallest read points intact, but
combine mvcc write number and seqId. This will allow cleaner semantics + implementation +
smaller data files. 
> We can do some brainstorming for 0.98. We still have to verify that this would be semantically
correct, it should be so by my current understanding.



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