Re: Low FD limit a performance issue?

From: Kurt <kurtengelo_at_gmail.com>
Date: Wed, 2 Nov 2011 16:31:45 -0700
Message-ID: <CAOKMGha0AJmKeKiSqKM+KNo6-2xoLeaiMxzxp1QKhFZKVV-WNw_at_mail.gmail.com>

Hi Dave,
I didn't have time to actually test this - so take it for what it's worth.

I believe what happens if you don't have a sufficient FD limit is that when the Oracle database hits the number of open files, it will close a currently open file and open the file it wanted to read.

In the past, that was a sufficiently great performance hit that someone decided to put that message out to the alert log. It would be a mildly interesting exercise to see if it would still be 'severe performance degradation'. Second interesting test would be if the FD limit were set to a wildly low limit - such as 4.

I hope this helps.
Kurt

On Wed, Nov 2, 2011 at 6:49 AM, Herring Dave - dherri < Dave.Herring_at_acxiom.com> wrote:

> I did a crazy thing the other day - reviewed an alert log from a system I
> inherited a while ago. I found that after a recent auto-restart a message
> was displayed: "WARNING:Oracle instance running on a system with low open
> file descriptor limit. Tune your system to increase this limit to avoid
> severe performance degradation.". Sure enough, on this system
> /etc/init.d/init.crsd has the line "ulimit -n unlimited" in it, which on
> RHEL 4 generates an error, so the FD limit defaults to 1024 (bug 5862719).
>
> The problem is easy to resolve but my question is on Oracle's warning.
> How could a low open file descriptor limit be a potential source of
> "severe performance degradation"? Isn't it a black-or-white issue, either
> the limit is high enough or if not, the db won't open/you can't add more
> datafiles?
>
> DAVID HERRING
> DBA
> Acxiom Corporation
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Received on Wed Nov 02 2011 - 18:31:45 CDT