Re: DNS causing performance issues

/etc/hosts entries may -- or may not -- take precedence over NFS. It depends on your OS and also on how it is configured. In some cases, you can configure your OS to ignore /etc/hosts entirely.

I am pretty sure that DNS can, indeed, cause performance issues for SQL*Plus connections -- at least when connecting over TCP/IP. It has been some time (> 2 years) since I have last experienced this, but I have seen SQL*Plus (and other) connections take many (> 10?) minutes to complete as the result of failed (and misconfigured) DNS servers. In the particular case that I recall, the "primary" DNS server used by the database host had failed, and the secondary (which was separated by a firewall) was not correctly configured for reverse-lookups on the IP range for the incoming connections.

Whatever the exact cause (it *was* more than 2 years ago), the net result was *very* long timeouts and eventual failures for reverse-IP resolution. Where exactly does Oracle rely on this? No sure precisely -- I would guess that it is probably the listener, reverse resolving the IPs for logging purposes. It is possible that the database *also* does reverse lookups to populate the MACHINE column for V$SESSION, however, I would *hope* that information is passed to the database by the listener.

Many other applications (e.g. webservers) do this also (for logging), and many provide the option of turning off the reverse-lookups for performance. I would like to think that such an option exists for the TNS listener, but I have never been sufficiently motivated to go look. I usually find it easier to make the sysadmins configure DNS correctly. :-)

On Mon, Mar 17, 2008 at 3:06 PM, Crisler, Jon <Jon.Crisler_at_usi.com> wrote:

> Since /etc/host entries override DNS, I don't see how this could have
> any affect, assuming that /etc/host is correct. Perhaps you had some
> routing or bandwidth congestion occurring ?
>
> -----Original Message-----
> From: oracle-l-bounce_at_freelists.org
> [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Dan Norris
> Sent: Monday, March 17, 2008 3:45 PM
> To: Oracle L
> Subject: DNS causing performance issues
>
> ...snip...
>

> The symptom reported to me was that the extract processes was slow. It
> usually takes about 45 minutes and ran for about 5 hours (it did
> complete successfully). Someone suggested that a previous performance
> problem was resolved by ensuring that proper reverse DNS records were in
>
> place. In this case, they found that those DNS records did not
> exist--nor did the forward records, but there were a proper /etc/hosts
> entries on the servers involved. This process runs once a day. After the
>
> performance issue, the DNS was updated to have proper forward and
> reverse DNS records. The next day, the extracts ran in about 32
> minutes--faster than before. I'm chalking it up to coincidence.
>
> ...snip...
>
> Dan
> --
> http://www.freelists.org/webpage/oracle-l
>
>
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>
>
>

-- 
Cheers,
-- Mark Brinsmead
Senior DBA,
The Pythian Group
http://www.pythian.com/blogs

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