SUMMARY: Your opinion on swap size

Wow.  I asked for "opinions" and got a bunch.  I really appreciate the time
everyone took to provide me with this insight and information.

In my personal experience, I have noticed that Solaris is brutally efficient
at memory management and Solaris 8's "cyclical page cache" does well at
reducing the need for large amounts of swap.  Cyclical page cache replaced
priority paging starting in Solaris 8.

The general consensus seems to be 1xRAM up to 4gig of physical RAM and 4gig
swap for any amount over 4gig of physical RAM.  I received every range of
answer from "none" to "2xRAM+whatever your apps can use".  Clive McAdam gave
the first reference to Sun's "official" recommendations, which while vague,
gives a good starting point.  The link is
http://docs.sun.com/db/doc/817-6960/6mmah94ck?q=swapfs&a=view  Between reading
this and other information, it seems that anything over 4 gig will not be
utilized, even in a crash situation, as the core dump will be essentially
"swapped" to disk in /var/crash/`uname -n`/ (or your designated crashdump
location) as it is created - provided that diskspace in the crash location is
adequate.

Kalyan Manchikanti gave an excellent link to a Sunsolve article with
additional information (Sunsolve login required to access this link):
http://sunsolve.sun.com/private-cgi/retrieve.pl?doc=infodoc%2F16987&zone_32=s
wap%2A%20size%2A%20

Russell Page gave good insight on his reasoning behind his personal core dump
strategy as well as an interesting historical note on how the 2xRAM rule got
started:

"Historically, the 2xRAM rule was an artifact of the virtual memory
architecture on a VAX. Vaxen couldn't demand page off the file system, so
loading a progam meant copying the image from the file system to the swap
area, then paged it in from there. So on a VAX, a large chunk of the swap
space was no longer available for swap, and so admins had to configure lots
of swap."

I also found some good information Googling.  One page of interest:
http://web.brandeis.edu/pages/view/Network/Solaris8CoreDumps

Thanks go out to everyone who responded.  While far too many responded to name
everyone, the following people provided the most detailed and helpful
answers:


Clive McAdam
Kalyan Manchikanti
Russell Page
Nicolas Dorfsman
Thomas M. Payerle
Jon Hudson


--
Robert Kracke
Southern Company, EMS
Systems Administration
Support@emss.com
postmaster@emss.com
--
----- Original Message -----
From: Robert Kracke
To: Sun Managers
Sent: Thursday, August 12, 2004 9:06 AM
Subject: Your opinion on swap size


Recently, we have been getting more midrange systems where in the past we have
settled for multiple small server systems (typically V480 and V880).  In the
past, I have used the simplistic 2xtotalRAM formula to determine a general
baseline for swap.  At present, the systems I am working with are starting in
the V1280 range, typically have 8 or 12 processors and a minimum of 16gig RAM.
I can't imagine a system would need 32gig of hard disk swap, but I may be
wrong.  I also realize that the intended use of the system plays into this
formula (i.e. simple web server vs. raw number crunching vs. big databases).
Can anyone point me to some basic guidelines or offer advice on an effective
and efficient swap formula?

Any assistance provided is appreciated and a summary of responses will
follow.
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