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https://issues.apache.org/jira/browse/HBASE-17471?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15833862#comment-15833862
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Allan Yang commented on HBASE-17471:
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Actually, we notice the performance regression after merging mvcc and sequence
id, too. I even prepared a patch which separate mvcc and seqid as before
HBASE-8763. This patch is based on our custom branch-1.1.2. It works well, and
have the same performance(maybe a little better) as preAssignMVCC. But consider
of many features are based on the fact that mvcc and seqid is combined, I gave
up the patch. If anyone interested in separate mvcc and sequence, you are
welcomed to discuss with me. I'd like to say, separating them can truly resolve
a lot of problems. Meanwhile, let's forces on this patch.
> 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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