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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-599) Improve Namenode robustness by prioritizing datanode heartbeats over client requests
Date Mon, 17 May 2010 17:37:50 GMT

    [ https://issues.apache.org/jira/browse/HDFS-599?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12868299#action_12868299
] 

dhruba borthakur commented on HDFS-599:
---------------------------------------

> You should break the current ClientProtocol into AdminProtocol and the real ClientProtocol

We can certainly do this.  The ClientProtocol essentially consists methods that fall into
two categories:

1. Calls that modify file system metadata in the namenode
2. Calls that retrieve portions of file system metadata from the namenode.

Let's consider the case when the NN is restarting and is in safemode.  Only the servicePort
is open at this time. The calls in category 1 will anyway fail because namenode is in safemode.
That leaves us with only the calls in category 2. When the namenode is in safemode, the admin
would still want the ability to be able to list files (dfs -lsr) , get status on files (dfs
-ls), see the amount of space used by a portion of the namespace (dfs -count), validate block
size of an existing file(s), look at target of a symlink. That means that admin would want
to invoke most of the calls in category 2, isn't it? If you agree with the above, then it
is not very beneficial to break up ClientProtocol into two parts, because both the parts would
have to be available on the service port?

There are quite a few things that we can do to handle "a mis-configured client happens to
choose the service port as its client port". if we do not even list the service port in the
client's config, that would be a good thing.... can we start there?




> 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: Dmytro Molkov
>         Attachments: HDFS-599.patch
>
>
> 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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