Re: DNS causing performance issues
Date: Mon, 17 Mar 2008 19:39:04 -0600
/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
> 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.
-- Cheers, -- Mark Brinsmead Senior DBA, The Pythian Group http://www.pythian.com/blogs -- http://www.freelists.org/webpage/oracle-lReceived on Mon Mar 17 2008 - 20:39:04 CDT