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From "Yu Li (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17471) Region Seqid will be out of order in WAL if using mvccPreAssign
Date Mon, 23 Jan 2017 04:00:29 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17471?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15833874#comment-15833874
] 

Yu Li commented on HBASE-17471:
-------------------------------

bq. I'd like to say, separating them can truly resolve a lot of problems.
Does this mean we need to fix more problems if stick on current mvcc-seqid-merge way? Please
tell us more about the problems. I think we need to compare a) the advantage of merging mvcc
and sequence-id and b) the efforts to fix all the problems introduced.

Ping [~stack], for your special notice sir. [~allan163] works in [~zjushch]'s team and use
HBase as a *common* service for Alibaba group, while my team using HBase as a *special* service
(focusing on search and recommendation) for Alibaba search, so they're experiencing more common
workloads/use-cases and may encounter more issues than us (such as in our scenario we don't
have append/increment requests but they do) in product env.

> 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, HBASE-17471.v3.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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