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From "Todd Lipcon (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 17:58:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2874?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13199031#comment-13199031
] 

Todd Lipcon commented on HDFS-2874:
-----------------------------------

bq. Adding any type of write ordering to edit log updates will increase commit latency. For
example, writing to shared edit log first, waiting for this to complete and then writing to
local dirs will double the write latency.

We already _do_ write to each volume serially, so the latency is no different. I agree there
is a potential optimization to do them in parallel, but since we've been living fine without
for years, I don't think it's on the critical path for most installations.

bq. A better design is to write all the locations in parallel and then make a policy decision
during recovery as to go with higher txid or the lower one.

Except that the remote NN can't see the local edit dirs, so no such policy exists.

                
> 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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