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From "Allan Yang (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HBASE-17471) Region Seqid will be out of order in WAL if using mvccPreAssign
Date Mon, 23 Jan 2017 02:16:26 GMT

     [ https://issues.apache.org/jira/browse/HBASE-17471?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Allan Yang updated HBASE-17471:
-------------------------------
    Attachment: HBASE-17471.v2.patch

Add a init patch to clarify my thought and run through the Hundson.
Thanks for [~Apache9] 's idea of removing the additional lock.
In this patch, I bring the mvcc preassign to the wal.append(), all acquirement of seqid and
ringbuffer's txid is synchronized under mvcc.writeQueue, making sure the seqid in WAL is increasing
monotonically. I also moved the action of stamping seqid to cell from ringbuffer's single
consume thread to increase throughput. (According to one of my test, acquire mvcc under lock
and stamping them to cell takes 25% time of the append action)
One thing to do is making preAssign configurable.
[~carp84] (the original author of preAssignMVCC) and the others, please give me some advice.

> 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
>            Priority: Critical
>         Attachments: HBASE-17471.patch, HBASE-17471.tmp, HBASE-17471.v2.patch
>
>
>  mvccPreAssign was brought 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 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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