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From "Phabricator (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5344) [89-fb] Scan unassigned region directory on master failover
Date Thu, 16 Feb 2012 05:53:01 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5344?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13209128#comment-13209128
] 

Phabricator commented on HBASE-5344:
------------------------------------

Karthik has commented on the revision "[jira] [HBASE-5344] [89-fb] Scan unassigned region
directory on master failover".

  @stack -

  << So, the '/unassigned' dir will change to be named '/regions' or some such? On region
open, we'll update its state in zk to be OPENED? And leave it there (and update .META. too).
We don't really need .META. then? Smile. >>

  Not totally... the idea is to change /unassigned to /regions/<tablename> and it will
not be deleted on the region being opened. This will only track assignment of regions though.
We still need meta to figure out start/stop keys and other information like preferred regionservers
for the regions. In order to truly eliminate meta, we should be willing to store more permanent
data in ZK, but thats probably not needed unless we are going all the way to rip out root
and meta special-casing, which would be a huge change.

  << "We would like to rely on ZK and (for now) on META instead to recover the region
assignment on master startup/failure." The hard part in here is on failover, what if the .META.
is on a crashed server? You'll need to process its logs and get .META. back online before
you can proceed w/ failover. To get it online, you'll need to listen for events (though I
suppose you could filter and only process .META. events). Or what if the the server carrrying
.META. crashes during onlining. >>

  The idea here is that the master can read unassigned (while continuing to queue events).
It will split all logs (via distributed log splitting) and process opening of the meta region
out of band (not go though the standard heartbeat based assignment - we do this for root upon
startup, we need something similar for meta).


REVISION DETAIL
  https://reviews.facebook.net/D1605

                
> [89-fb] Scan unassigned region directory on master failover
> -----------------------------------------------------------
>
>                 Key: HBASE-5344
>                 URL: https://issues.apache.org/jira/browse/HBASE-5344
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Mikhail Bautin
>            Assignee: Mikhail Bautin
>         Attachments: D1605.1.patch
>
>
> In case the master dies after a regionserver writes region state as OPENED or CLOSED
in ZK but before the update is received by master and written to meta, the new master that
comes up has to pick up the region state from ZK and write it to meta. Otherwise we can get
multiply-assigned regions.

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