Re: How much RAM is to much

Andrew et. al,

The percentage depends on whether or not direct I/O is configured and is working 
per specification. If you have direct I/O properly configured, in the big 
picture,  the consumption of memory by the filesystem buffer cache will not 
affect your memory consumption numbers. But if you do NOT have direct I/O 
configured and depending on your operating system (Linux vs. Unix), the issue 
then is what are the ceilings setup for the filesystem buffer cache's memory 
consumption. The last time I checked there is no equivalent of "bufhwm" 
(Solaris) or file_cache_max_pct (HP-UX) on Linux. Which means that if you don't 
have direct I/O configured on Linux (which btw is not good practice), you can be 
pretty much guaranteed that up to 100% of configured memory can be utilized by 
the OS for the filesystem buffer cache. There have been many customer cases in 
the past few years, where the lack of direct I/O has caused significant 
paging/swapping overhead. The lack of direct I/O will also increase "sys" CPU 
utilization and causing unnecessary overhead and contention on the system. Not 
at all worth it!

Bottom line - please enable direct I/O, make sure it is working (by performing 
the relevant truss, strace etc) and then finalize the memory allocations for 
your SGAs & PGAs.


Cheers,

Gaja
 Gaja Krishna Vaidyanatha,
Founder/Principal, DBPerfMan LLC
http://www.dbperfman.com
Phone - 001-(650)-743-6060
Co-author:Oracle Insights:Tales of the Oak Table - 
http://www.apress.com/book/bookDisplay.html?bID=314
Co-author:Oracle Performance Tuning 101 - 
http://www.amazon.com/gp/reader/0072131454/ref=sib_dp_pt/102-6130796-4625766




________________________________
From: Andrew Kerber <andrew.kerber_at_gmail.com>
To: RStorey_at_dcso.nashville.org
Cc: oracle-l-freelists <oracle-l_at_freelists.org>
Sent: Thu, February 10, 2011 11:18:28 AM
Subject: Re: How much RAM is to much

I generally use the rule of thumb for Linux/unix of oracle can have up  to 80% 
of the RAM on the system on a dedicated server. However, make sure everything on 
the OS is configured per the installation instructions for oracle before you 
start dedicating all those resources to oracle.


On Thu, Feb 10, 2011 at 1:02 PM, Storey, Robert (DCSO) 
<RStorey_at_dcso.nashville.org> wrote:

So, I’m moving my 9i 32 bit database to a 10g 64 bit database.  My 9i box has 4 
gig of ram and the usual 23bit limitations. My SGA and such on the 9i box 
probably hovers around 1.2gig.
> 
>I have 24 gig of ram on the new box.  From a data aspect, that will darn near 
>load my entire database to memory.
> 
>So, in setting SGA_TARGET, how much is too much?  Before I was told the box 
>specs, I was thinking 3 gig.  But, with 24 gig available, and I’m the ONLY 
>application on the box….how much is to much?
> 
>What are the benefits and cons to setting this value at say, 12gig, with a 
>SGA_MAX value of 15G.
> 
> 


-- 
Andrew W. Kerber

'If at first you dont succeed, dont take up skydiving.'

--
http://www.freelists.org/webpage/oracle-l