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From Allen Wittenauer <awittena...@linkedin.com>
Subject Re: editing etc hosts files of a cluster
Date Mon, 19 Oct 2009 23:17:03 GMT



On 10/19/09 11:46 AM, "Edward Capriolo" <edlinuxguru@gmail.com> wrote:

> I am interested in your post. What has caused you to run caching DNS
> servers on each of your nodes? Is this a hadoop specific problem or a
> problem  specific to your implementation?

Hadoop does a -tremendous- amount of hostname lookups.  If you don't have
either nscd or a local DNS caching server, you are likely throwing what
could be some significant performance gains away.

> My assumption here is that a hadoop cluster of say 1000 nodes would
> repeatedly talk to the same 1000 nodes.

... and that's the catch!  Every node running the DFSClient code or being
called out from a map/reduce task is a potential hostname that would need be
resolved.  Just think about something like distcp.

Also note that this is before we talk about monitoring, any other naming
services, CNAMEs, multi-As, etc, that get built as a normal part of running
an infrastructure.

> Are you saying that nscd is
> inadequacy to handle the size of the cache, or nscd is not very
> efficient? What exactly is the reason you are running a caching DNS
> server on each node?

In the case of Yahoo!, we had (or, at least, a perception) that we had or
were going to have jobs that did a lot of direct DNS lookups and/or
accessed/referenced things outside of the local grid.  Also note that a DNS
caching server is going to store more information about hostnames than a
simple host to IP service like nscd.

Hypothetical:  Let's say I'm building rules for a spam filter and part of my
process is to look up the MX record for a given host.  nscd isn't going to
help you there.

In the case of LinkedIn, the jury is still out.  I suspect we don't have
nscd.conf tuned correctly.  Our grid isn't that big, our connections in/out
are fairly small, etc. It has been one of the things on my todo list since I
got hired here 2 months ago. :)

[For the record, I'm not one of those crazy people who turns off nscd
because I had a bad experience with a  broken version five years ago.  In
the case of Yahoo!, I was the crazy person who started insisting we turn it
on, albeit not for hosts.]


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