RE: Low FD limit a performance issue?

Oracle Support article
Open Files/Open File Descriptors [ID 787780.1]

explains the difference between file-descriptors and open files

 
Hemant K Chitale

-----Original Message-----

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Herring Dave - dherri Sent: Friday, November 04, 2011 1:48 AM
To: Kurt
Cc: oracle-l List
Subject: RE: Low FD limit a performance issue?

Currently the base is 1024 and we're working with ASM, so in the end we'd only have to worry if somehow a database process needed to open more than 1024 non-ASM database files, which I can't imagine how that'd ever be the case in our environment. Maybe this only applies to non-ASM dbs. DAVID HERRING
DBA
Acxiom Corporation
EML dave.herring_at_acxiom.com
TEL 630.944.4762
MBL 630.430.5988
1501 Opus Pl, Downers Grove, IL 60515, USA WWW.ACXIOM.COM<http://www.acxiom.com/>

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From: Kurt [mailto:kurtengelo_at_gmail.com] Sent: Wednesday, November 02, 2011 6:32 PM To: Herring Dave - dherri
Cc: oracle-l List
Subject: Re: Low FD limit a performance issue?

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<mailto: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
EML dave.herring_at_acxiom.com<mailto:dave.herring_at_acxiom.com> TEL 630.944.4762<tel:630.944.4762>
MBL 630.430.5988<tel:630.430.5988>
1501 Opus Pl, Downers Grove, IL 60515, USA WWW.ACXIOM.COM<http://WWW.ACXIOM.COM>

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http://www.freelists.org/webpage/oracle-l Received on Thu Nov 03 2011 - 20:48:11 CDT