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From "Varun Sharma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4721) Speed up lease/block recovery when DN fails and a block goes into recovery
Date Thu, 25 Apr 2013 00:27:14 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4721?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13641222#comment-13641222
] 

Varun Sharma commented on HDFS-4721:
------------------------------------

One more thing, currently we do it this way:

1) Client is writing to File F1 (a datanode livees on the client)
2) Client+datanode on the machine crash
3) F1 is renamed to F2 - I don't think we try to recover lease here
4) After the rename is done, we call recover lease on the file

And then the sequence of incidents above - do you think rename is the culprit here...
                
> Speed up lease/block recovery when DN fails and a block goes into recovery
> --------------------------------------------------------------------------
>
>                 Key: HDFS-4721
>                 URL: https://issues.apache.org/jira/browse/HDFS-4721
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>    Affects Versions: 2.0.3-alpha
>            Reporter: Varun Sharma
>             Fix For: 2.0.4-alpha
>
>         Attachments: 4721-hadoop2.patch, 4721-trunk.patch, 4721-trunk-v2.patch, 4721-v2.patch,
4721-v3.patch, 4721-v4.patch, 4721-v5.patch, 4721-v6.patch, 4721-v7.patch, 4721-v8.patch
>
>
> This was observed while doing HBase WAL recovery. HBase uses append to write to its write
ahead log. So initially the pipeline is setup as
> DN1 --> DN2 --> DN3
> This WAL needs to be read when DN1 fails since it houses the HBase regionserver for the
WAL.
> HBase first recovers the lease on the WAL file. During recovery, we choose DN1 as the
primary DN to do the recovery even though DN1 has failed and is not heartbeating any more.
> Avoiding the stale DN1 would speed up recovery and reduce hbase MTTR. There are two options.
> a) Ride on HDFS 3703 and if stale node detection is turned on, we do not choose stale
datanodes (typically not heart beated for 20-30 seconds) as primary DN(s)
> b) We sort the replicas in order of last heart beat and always pick the ones which gave
the most recent heart beat
> Going to the dead datanode increases lease + block recovery since the block goes into
UNDER_RECOVERY state even though no one is recovering it actively. Please let me know if this
makes sense. If yes, whether we should move forward with a) or b).
> Thanks

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