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From "Jimmy Xiang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8797) Prevent merging regions from moving during online merge
Date Tue, 02 Jul 2013 18:21:29 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13698065#comment-13698065
] 

Jimmy Xiang commented on HBASE-8797:
------------------------------------

bq. Just pass through?
Stack, good catch.  I was thinking about it too last night.  We should update the state to
merging here.

bq. Should this RS_ZK_REGION_MERGE be called RS_ZK_REGION_MERGED ?
Yes, I agree. But that's some existing code. I will fix it anyway since I need to post another
patch.

bq. In isInStateForMerging, you are testing isOpenOrMergingOnServer. That seems a little loose?
Should it just be testing isMergingOnServer only?
Depends. For the first time, we need to check if it's OpenOnServer.  Later, we should check
MergingOnServer only.  Since a region can't be both open and merging on the same server, such
a checking is enough.

bq. So, if RS crashes after putting up two MERGING znodes, they are ephemeral (are they?)
and they will just evaporate? When the logs for crashed server are played, we will just presume
the merge did not succeed? And will assign out the two regions that were merging? Is there
a hole in here where the merge succeeded but master was not notified yet? In this case will
we find the merged region and assign it out?
Yes, the merging znode is ephemeral. If the RS crashes, the znode should be gone. There could
be a hole here. We need to figure out whether we should assign out the two merging regions,
or the new merged region.

bq. If we get the merge message and then that the RS has crashed, ofter WAL split, we'll assign
out the merged region .. and not the two daughter inputs?
If we get the merge message, the two merging regions are already removed from meta and the
new region is already inserted into meta. The two daughter inputs won't be assigned out any
more.

                
> Prevent merging regions from moving during online merge
> -------------------------------------------------------
>
>                 Key: HBASE-8797
>                 URL: https://issues.apache.org/jira/browse/HBASE-8797
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>            Reporter: Jimmy Xiang
>            Assignee: Jimmy Xiang
>         Attachments: trunk-8797.patch
>
>
> When two regions are merged online, they are closed but master doesn't know they should
stay closed during the merge.  If master moves them by mistake, for example, load balancer
kicks in, the merge could be messed up.

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