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From "Ivan Kelly (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1623) High Availability Framework for HDFS NN
Date Fri, 18 Mar 2011 10:57:29 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13008395#comment-13008395

Ivan Kelly commented on HDFS-1623:

I assume that in what is envisioned, the code running on active and standby will be identical,
with the only difference being whether an active switch is enabled. 

It would be good if the code for active/standby detection was pluggable. So that different
options for failover could be provided. It wouldn't be good to require that a zookeeper ensemble
be set up just to run a namenode. 

How does heartbeat deal with network partition? My understanding of it is that it sends packets
at intervals to the other node, and if they don't get through it considers the other dead.
This could create a situation where both active and standby think that the other is dead,
and both become active, leading to divergent filesystem states on each machine.

Also, the design indicates that more than 2 NN is out of scope. Why? Surely it's as easy to
design for N namenodes as it is for 2 namenodes.

If you want manual failover, from the server perspective you need to do nothing. Operators
can have 2 namenode machines, with the namenode only running on one, writing to shared storage.
When the want to failover to the standby they just have to ensure that the active is down
and start the namenode daemon on the standby.

I proposed a design last week for streaming updates from an active to a standby, it may be
interesting to you (ZOOKEEPER-1016). It does have some mentions of active/standby detection,
which I should remove. It occurs to me now, that this functionallity should be separated out
completely from the WALing and should live at the level of NameNode.java.

> High Availability Framework for HDFS NN
> ---------------------------------------
>                 Key: HDFS-1623
>                 URL: https://issues.apache.org/jira/browse/HDFS-1623
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Sanjay Radia
>            Assignee: Sanjay Radia
>         Attachments: Namenode HA Framework.pdf

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