[Hadoop-common-user] decomission a node

Inside the hdfs conf,

<property>
<name>dfs.hosts.exclude</name>
<value></value>
<description>Names a file that contains a list of hosts that are
not permitted to connect to the namenode. The full pathname of the
file must be specified. If the value is empty, no hosts are
excluded.</description>
</property>

Point this property at a file containing a list of nodes you want to
decommision. From there, use the command line "hadoop dfsadmin
-refreshNodes".

Alan,

You shouldn't need to worry about moving the data blocks off your node, assuming you're replicating your blocks 3x times. (I think its the default...)

You can bring down your node, and within 15 mins, Hadoop will recognize that node as down...

I think if you do a $> hadoop fsck / your system will recognize that those blocks are under replicated and will replicate the blocks on another machine.

If you're decommissioning ?sp? a node, then you're taking the machine out of the cluster permanently.

I'm also not sure how dropping a node will test the scalability. You would be testing resilience.

HTH

-Mike

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He's testing scale down, not scale up (which is the way we normally think of things... I was confused by the wording too).

In other words, "if I drop a node, how much of a performance hit is my job going to take?"

Also, for this type of timing/testing, I'd probably make sure speculative execution is off. It will likely throw some curve balls into the time.

Yes the effect of "scaling down" was the first thing I wanted to look at.
To process X GB it currently takes Y seconds with Z nodes.
If I process X GB with Z/2 nodes, does it take Y/2 seconds?
How about Z-1,Z-2,Z-3,.... nodes?

Right now my MR job process alot of small files (2000 files, @2.5MB each)
individually, so the next test would involve changing my MR job to combine
the small files into bigger pieces (closer to hdfs block size) and see
if that
is more effective.

Each line of my small files has a timestamp column and 55 columns with
numerical data and my reducer needs to calc the column averages for
certain time periods (last day, last hour,etc.) based on the timestamp.

Alan

This is pretty predictable.
Determine the average time it takes to process a m/r task.
If you can process 100 m/r tasks simultaneously, and then cut that to 50 m/r tasks you can handle simultaneously, your job will take twice as long to run.

Granted this will give you a rough estimate of how long it will take your job to run.

HTH

-Mike
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