[CentOS] Re: Apparently no swap configured

Christopher Chan christopher.chan at bradbury.edu.hk
Thu Oct 30 00:36:43 UTC 2008


Stephen Harris wrote:
> On Wed, Oct 29, 2008 at 09:56:34AM -0400, James B. Byrne wrote:
>> I believe that the current recommendation is 2 x physical memory up to 2
>> GB and then 1 x physical memory thereafter.
>>
>> See:
>> http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.2/html/Deployment_Guide/s1-swap-what-is.html
>>
>> " Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and
>> then an additional 1x physical RAM for any amount above 2 GB, but never
>> less than 32 MB.
> 
> That's a silly recommendation, and never been true for Linux.  RedHat
> don't always know what they're talking about.
> 
> In older BSD systems (eg around SunOS 4 times or before) swap space was
> utilised oddly; all memory was allocated from swap, so you needed _at
> least_ <physmem> of swap just to use all your real memory!  So if you
> added <physmem> of swap then your total virtual memory size was still
> only <physmem>.  No help!  So the rule of thumb came along that said
> "swap = 2*<physmem>" and that gave you a VM of 2*<physmem>.   Linux never
> did this (and I don't think modern BSDs do, either) so adding <physmem>
> of swap will automatically give you twice <physmem> of VM.

Ah, yes, not an older release of Solaris...SunOS4

A certain release of SunOS4 required swap be twice the amount of RAM or 
it would crash according to an article that I read but I cannot find 
right now. Here is a FAQ article related to SunOS that tells us how 
SunOS4 handled virtual memory (swap must be at least the same amount of 
RAM): http://www.bjnet.edu.cn/sun-admin/FAQ/F-comp-sys-sun/Q30-0.html

The source of this 'recommendation' that swap be twice that of RAM is 
affects more than just Linux

http://blogs.sun.com/jimlaurent/entry/solaris_faq_myths_and_facts

So please, stop spreading this swap should be twice the amount of RAM 
installed nonsense.

/me gets off soapbox.


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