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From "Uma Maheswara Rao G (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2874) HA: edit log should log to shared dirs before local dirs
Date Thu, 02 Feb 2012 05:43:53 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2874?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13198546#comment-13198546
] 

Uma Maheswara Rao G commented on HDFS-2874:
-------------------------------------------

{quote}
It occurs to me that using the "required" dirs as a proxy for the shared dirs isn't necessarily
correct. It's true that the shared dir should always be marked required, but an admin might
also configure some local or other dirs to be required.
{quote}
I too agree with Aaron on this point.

I am just thinking, can't we maintain priority level based lists, while updating jouranal
itself we can update in that order. Also we can provide iterator based on priority level.
Always shared jouranals should have high priority. Journals which are required and non shared
can be the next priority..etc?

                
> HA: edit log should log to shared dirs before local dirs
> --------------------------------------------------------
>
>                 Key: HDFS-2874
>                 URL: https://issues.apache.org/jira/browse/HDFS-2874
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: ha, name-node
>    Affects Versions: HA branch (HDFS-1623)
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Critical
>         Attachments: hdfs-2874.txt
>
>
> Currently, the NN logs its edits to each of its edits directories in sequence. This can
produce the following bad sequence:
> - NN accumulates 100 edits (tx 1-100) in the buffer. Writes and syncs to local drive,
then crashes
> - Failover occurs. SBN takes over at txid=1, since txid 1 never got writen.
> - First NN restarts. It reads up to txid 100 from its local directories. It is now "ahead"
of the active NN with inconsistent state.
> The solution is to write to the shared edits dir, and sync that, before writing to any
local drives.

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