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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-1975) HA: Support for sharing the namenode state from active to standby.
Date Sun, 27 Nov 2011 22:50:43 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1975?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13158067#comment-13158067

Todd Lipcon commented on HDFS-1975:

bq. How about update the generation stamp after logging the new block

The issue is that we need the new block to have the new genstamp, so we have to call {{nextGenerationStamp}}
before we create the new block. Alternatively, we could split up {{nextGenerationStamp}} into
two parts -- one which increments, and another which logs. But then we may have an issue if
there is a crash so the edit log includes the block allocation but not the SET_GENERATION_STAMP.

I've temporarily worked around this by changing the FSEditLogLoader code to only call {{notifyGenStampUpdate}}
in {{OP_ADD}}, not in {{OP_SET_GENSTAMP}}. So, the new block is added to the block manager
before it's notified as OK to handle the DN messages.

*BUT* -- there's another issue with using genstamps as our "gating" mechanism for the DN messages
- comment to follow.
> HA: Support for sharing the namenode state from active to standby.
> ------------------------------------------------------------------
>                 Key: HDFS-1975
>                 URL: https://issues.apache.org/jira/browse/HDFS-1975
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: name-node
>            Reporter: Suresh Srinivas
>            Assignee: Jitendra Nath Pandey
>         Attachments: HDFS-1975-HA.2.patch, HDFS-1975-HA.patch, HDFS-1975-HDFS-1623.patch,
HDFS-1975-HDFS-1623.patch, hdfs-1975.txt, hdfs-1975.txt
> To enable hot standby namenode, the standby node must have current information for -
namenode state (image + edits) and block location information. This jira addresses keeping
the namenode state current in the standby node. To do this, the proposed solution in this
jira is to use a shared storage to store the namenode state. 
> Note one could also build an alternative solution by augmenting the backup node. A seperate
jira could explore this.

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