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From "Jesse Yates (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6440) Support more than 2 NameNodes
Date Tue, 13 Jan 2015 17:33:36 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6440?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14275579#comment-14275579

Jesse Yates commented on HDFS-6440:

thanks for the comments.  I'll work on a new version, but in the meantime, some responses:
bq. StandbyCheckpointer#activeNNAddresses
The standby checkpointer doesn't necessarily run just on the SNN - it could be in multiple
places. Further, I think you are presupposing that there is only one SNN and one ANN; since
there will commonly be at least 3 NNs, any one of the two other NNs could be the active NN.
I could see it being renamed as potentialActiveNNAddresses, but I don't think that gains that
much more clarity for the increased verbosity.

bq.  I saw you removed {final}
I was trying to keep in the spirit of the original mini-cluster code. The final safety concern
is really only necessary in this case when you are changing the number of configured NNs and
then accessing them in different threads; I have no idea when that would even make sense.
Even then you wouldn't have been thread-safe in the original code as it there is no locking
on the array of NNs. I removed the finals to keep the same style as the original wrt to changing
the topology.

bq. Are the changes in 'log4j.properties' necessary?

Not strictly, but its just the test log4j properties (so no effect on the production version)
and just adds more debugging information, in this case, which thread is actually making the
log message.

I'll update the others

> Support more than 2 NameNodes
> -----------------------------
>                 Key: HDFS-6440
>                 URL: https://issues.apache.org/jira/browse/HDFS-6440
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: auto-failover, ha, namenode
>    Affects Versions: 2.4.0
>            Reporter: Jesse Yates
>            Assignee: Jesse Yates
>         Attachments: Multiple-Standby-NameNodes_V1.pdf, hdfs-6440-cdh-4.5-full.patch,
hdfs-6440-trunk-v1.patch, hdfs-multiple-snn-trunk-v0.patch
> Most of the work is already done to support more than 2 NameNodes (one active, one standby).
This would be the last bit to support running multiple _standby_ NameNodes; one of the standbys
should be available for fail-over.
> Mostly, this is a matter of updating how we parse configurations, some complexity around
managing the checkpointing, and updating a whole lot of tests.

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