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https://issues.apache.org/jira/browse/HDFS-4721?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13638125#comment-13638125
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Varun Sharma commented on HDFS-4721:
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nicholas,
Good point. I will create a patch which has that.
Ted,
As far as I know, the last WAL block is in UNDER_CONSTRUCTION state and we call
internalReleaseLease. When we call internalReleaseLease, it goes into
UNDER_RECOVERY state and the namenode enqueues it for recovery to the primary
DataNode. Namenode actually communicates this enqueued recovery block to the
primary datanode during heart beat. Now lets say the primary never heart beated
or was lost. We ideally want to retry recoverLease in that case if the lease
has *not* been recovered. I guess the best way around it would be to have an
API for checking if lease is recovered or not. Right now, we have an API to do
that but it also unconditionally enqueues a block for recovery.
> 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
>
>
> 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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