[Gllug] Re: Swap Restrictions

[Daniel P. Berrange]

On Mon, Apr 17, 2006 at 08:05:50AM +0100, Steve Nelson wrote:
> On 4/12/06, Steve Nelson <sanelson at gmail.com> wrote:
> > Hello All,
> >
> > I was always under the impression that on x86 architecture there is a
> > 2G limit of size of swap areas.

No there hasn't been a 2 GB limit for quite a long time now. This is,
however, alot of incorrect documentation on this fact :-)

> 
> A colleague has pointed me to the following article in redhat magazine:
> 
> http://www.redhat.com/magazine/017mar06/departments/tips_tricks/
> (right at the bottom)
> 
> It states:
> 
> """In Red Hat Enterprise Linux 3 and above, the mkswap command allows
> the creation of a swap partition of a larger size than 2GB."""
> 
> Does anyone know is this is due to a newer version of mkswap? Or is
> this a Redhat patch?
> 
> My mkswap(8) man page is dated 1999, and the writer of the article
> states that documentation is being updated.

This looks like it was an upstream change in the kernel - my 'mkswap'
man page on FC4 says:

  "The  maximum  useful  size  of a swap area depends on the 
   architecture and kernel version.  It is roughly 2GiB on 
   i386, PPC, m68k, ARM, 1GiB on sparc, 512MiB on mips, 128GiB 
   on alpha and 3TiB on sparc64. For kernels after 2.3.3 there 
   is no that limitation."

> I have been able to make >2G swap partitions on RHAS 3, although on
> vmware ESX 2.0 I can't.  The difference seems to be that vmware is
> running:

Now the more interesting question, is whether you actually want / need
> 2 GB of swap. I'd argue that by the time a server has used up 2 GB
of swap, the VM will be thrashing like hell & be unable to do enough
any useful 'work'. Sure, it might eventually come out of swap death,
but not before being effectively offline for so long that the SLA for
any app running is long since broken.

> # rpm -q util-linux
> util-linux-2.11n-12.7.3
> 
> Redhat box is running:
> 
> # rpm -q util-linux
> util-linux-2.11y-31.11
> 
> This suggests that vmware is using an old version:
> 
> [root at s3td-a1]/local# rpm -qiv util-linux | grep -Ei "build date|packager"
> Release     : 12.7.3                        Build Date: Mon 24 Jun
> 2002 03:30:23 PM BST
> Packager    : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
> 
> So what happened to mkswap in that time?

That's anybodies guess - util-linux has a wierd version numbering
scheme, so what above looks like a relatively minor update can contain
a huge range of changes.

Regards,
Dan.
-- 
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|=-   berrange at redhat.com  -  Daniel Berrange  -  dan at berrange.com    -=|
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