[Gllug] Linux - big not small

[Richard Jones]

On Thu, Aug 04, 2005 at 03:21:24PM +0100, John Southern wrote:
> When memory was smaller say 32 or 64MB, the documentation always said swap 
> should be twice the RAM.

The whole "swap must be twice as large as physical RAM" thing is based
on an ancient version of Unix which required that all physical RAM was
backed by swap first, before it could use further swap for actually
swapping.  On this ancient system, it was a requirement to have swap
>= RAM, and having swap = 2 * RAM would effectively double available
memory.  This has never applied to Linux, although that hasn't stopped
people from parroting the rule frequently ever since.

I've seen database systems run without swap at all, on the basis that
it was far better for the machine to crash, than for it to sit their
swapping and processing queries excruciatingly slowly.

> As Ram increases into the Gigabyte realms, is there a limit for
> swap?

I don't know the definitive answer to this.  I _think_ that the
current situation is 2GB max per swapfile, and up to MAX_SWAPFILES
(=32 by default) swapfiles.  Perhaps PAE is a factor too ...  On x86
anyway.

Rich.

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Richard Jones, CTO Merjis Ltd.
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