[CentOS] Re: Apparently no swap configured

John R Pierce pierce at hogranch.com
Thu Oct 30 02:07:03 UTC 2008

Stephen Harris wrote:
> 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>.  

<old codger hat>  this was also how early IBM System/370 DOS/VS 
worked...  the swap space (which was contiguous cylinders of the disk, 
btw) was mapped 1:1 to virtual memory address space </old codger hat>

now, on late model Solaris, 'swap' is also used as tmpfs, which /tmp and 
/var/run utilizes by default, so you definitely want to allocate 
sufficient swap space for this at least, regardless of how large your 
physical memory is.

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