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Home -> Community -> Mailing Lists -> Oracle-L -> RE: [Oracle-L][Long...] SHMMNI choice affect SHMMAX choice?
You can certainly have more than one memory segment for your SGA, in Linux at least. However, fragmenting was bad in traditional Unix - Ive not tested the performance impact for Linux.
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Joel.Patterson_at_crowley.com
Sent: 20 December 2006 13:10
To: goodjobfastcar_at_gmail.com; Oracle-L_at_freelists.org
Subject: RE: [Oracle-L][Long...] SHMMNI choice affect SHMMAX choice?
I believe shmmax is contiguous memory. Ie The SGA has to be contiguous. if shmmax is 2gbs, you could have a database with 1GB SGA, or one with 2GBs, but not one with 3Gbs.
Joel Patterson
Database Administrator
joel.patterson_at_crowley.com
x72546
904 727-2546
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Peter Smith
Sent: Tuesday, December 19, 2006 4:44 PM
To: Oracle-L_at_freelists.org
Subject: [Oracle-L][Long...] SHMMNI choice affect SHMMAX choice?
People,
I've been asked to document how a DBA should set values of some Linux kernel parameters.
This is an easy task since it is a topic covered in the Oracle installation documents.
While writing my document, my curiosity triggered some simple questions.
What is the purpose of SHMMNI?
If I set SHMMNI to a larger value,
can I lower SHMMAX to a value to support a given SGA?
I found this description of SHMMNI via google:
SHMMNI sets the maximum number of shared memory segments which can be allocated at any one time.
The above description suggests that I can carve up my shared memory allocation into a bunch of smaller pieces and I would be able to have SHMMAX be a smaller number as I increase the value of SHMMNI.
Is this a correct understanding?
The Oracle docs suggest a SHMMNI starting value of 4096.
If I want an SGA sized at 2GB,
and if I have a Linux box with 3GB total RAM and 2GB RAM available for the SGA,
and if I have SHMMNI set to 4096,
can I theoretically have SHMMAX set to a small value like
(2 * 1024 * 1024 * 1024 ) / 4096 = 524288
??
By the way here is the document I wrote up which discusses some kernel parameters.
I'd welcome any comments (even if they are mean!) from DBA/SysAdmins out there.
http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html
-- http://www.freelists.org/webpage/oracle-lReceived on Wed Dec 20 2006 - 10:34:39 CST
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