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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3885) QJM: optimize log sync when JN is lagging behind
Date Thu, 06 Sep 2012 21:50:08 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13450085#comment-13450085
] 

Todd Lipcon commented on HDFS-3885:
-----------------------------------

Another good idea, Chao.

I'm going to quickly implement the simpler idea described in the "description" field of this
JIRA, since it's trivial to do so. We can then open another JIRA to actually aggregate larger
batches "in the queue" to avoid the round trip times.
                
> QJM: optimize log sync when JN is lagging behind
> ------------------------------------------------
>
>                 Key: HDFS-3885
>                 URL: https://issues.apache.org/jira/browse/HDFS-3885
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>    Affects Versions: QuorumJournalManager (HDFS-3077)
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>
> This is a potential optimization that we can add to the JournalNode: when one of the
nodes is lagging behind the others (eg because its local disk is slower or there was a network
blip), it receives edits after they've been committed to a majority. It can tell this because
the committed txid included in the request info is higher than the highest txid in the actual
batch to be written. In this case, we know that this batch has already been fsynced to a quorum
of nodes, so we can skip the fsync() on the laggy node, helping it to catch back up.

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