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Date: Mon, 16 Jan 2017 01:19:26 +0000 (UTC)
From: "Allan Yang (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
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Subject: [jira] [Commented] (HBASE-17471) Region Seqid will be out of order
 in WAL if using mvccPreAssign
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archived-at: Mon, 16 Jan 2017 01:19:47 -0000


    [ https://issues.apache.org/jira/browse/HBASE-17471?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15823338#comment-15823338 ] 

Allan Yang commented on HBASE-17471:
------------------------------------

Thank you, Ted, have changed the test name and the path name. Sorry that, this UT is not a 'real' UT, it will run forever just used to point out this issue can happen. In the current system, disorder in WAL will not address a exception, and it will not fail the writes. So I have to assert it to log this scenario. A new UT against this situation will need more think.

> Region Seqid will be out of order in WAL if using mvccPreAssign
> ---------------------------------------------------------------
>
>                 Key: HBASE-17471
>                 URL: https://issues.apache.org/jira/browse/HBASE-17471
>             Project: HBase
>          Issue Type: Bug
>          Components: wal
>    Affects Versions: 2.0.0, 1.4.0
>            Reporter: Allan Yang
>            Assignee: Allan Yang
>         Attachments: HBASE-17471.tmp
>
>
>  mvccPreAssign was bring by HBASE-16698, which truly improved the performance of writing, especially in ASYNC_WAL scenario. But mvccPreAssign was only used in {{doMiniBatchMutate}}, not in Increment/Append path. If Increment/Append and batch put are using against the same region in parallel, then seqid of the same region may not monotonically increasing in the WAL. Since one write path acquires mvcc/seqid before append, and the other acquires in the append/sync consume thread.
> The out of order situation can easily reproduced by a simple UT, which was attached in the attachment. I modified the code to assert on the disorder: 
> {code}
>     if(this.highestSequenceIds.containsKey(encodedRegionName)) {
>       assert highestSequenceIds.get(encodedRegionName) < sequenceid;
>     }
> {code}
> I'd like to say, If we are allow disorder in WALs, then this is not a issue. 
> But as far as I know, if {{highestSequenceIds}} is not properly set, some WALs may not archive to oldWALs correctly.
> which I haven't figure out yet is that, will disorder in WAL cause data loss when recovering from disaster? If so, then it is a big problem need to be fixed.
> I have fix this problem in our costom1.1.x branch, my solution is using mvccPreAssign everywhere, making it un-configurable. Since mvccPreAssign it is indeed a better way than assign seqid in the ringbuffer thread while keeping handlers waiting for it.
> If anyone think it is doable, then I will port it to branch-1 and master branch and upload it. 
>  


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