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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-599) Improve Namenode robustness by prioritizing datanode heartbeats over client requests
Date Wed, 16 Sep 2009 22:27:57 GMT

    [ https://issues.apache.org/jira/browse/HDFS-599?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12756258#action_12756258
] 

Todd Lipcon commented on HDFS-599:
----------------------------------

bq. One simpler solution could be to consider average heart beat time across all the datanodes
before marking one 'dead':

Average is not robust for this - if you have a couple of dead datanodes they'll skew the mean
up. What you really want is to detect high outliers. The traditional method to find high outliers
is to find the interquartile range (75th percentile - 25th percentile), and then any which
is 1.5*IQR greater than the 75th percentile point is considered a high outlier. The datapoints
for this calculation should probably be set to "lateness" (defined as max(0, msSinceLastHeartbeat
- expectedHeartbeatInterval))

A more complicated but very effective failure detection mechanism which is used by Cassandra
is the Phi Accrual Failure Detector: http://www2.computer.org/portal/web/csdl/doi/10.1109/RELDIS.2004.1353004

> Improve Namenode robustness by prioritizing datanode heartbeats over client requests
> ------------------------------------------------------------------------------------
>
>                 Key: HDFS-599
>                 URL: https://issues.apache.org/jira/browse/HDFS-599
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>
> The namenode processes RPC requests from clients that are reading/writing to files as
well as heartbeats/block reports from datanodes.
> Sometime, because of various reasons (Java GC runs, inconsistent performance of NFS filer
that stores HDFS transacttion logs, etc), the namenode encounters transient slowness. For
example, if the device that stores the HDFS transaction logs becomes sluggish, the Namenode's
ability to process RPCs slows down to a certain extent. During this time, the RPCs from clients
as well as the RPCs from datanodes suffer in similar fashion. If the underlying problem becomes
worse, the NN's ability to process a heartbeat from a DN is severly impacted, thus causing
the NN to declare that the DN is dead. Then the NN starts replicating blocks that used to
reside on the now-declared-dead datanode. This adds extra load to the NN. Then the now-declared-datanode
finally re-establishes contact with the NN, and sends a block report. The block report processing
on the NN is another heavyweight activity, thus casing more load to the already overloaded
namenode. 
> My proposal is tha the NN should try its best to continue processing RPCs from datanodes
and give lesser priority to serving client requests. The Datanode RPCs are integral to the
consistency and performance of the Hadoop file system, and it is better to protect it at all
costs. This will ensure that NN  recovers from the hiccup much faster than what it does now.

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