Re: [Xen-users] Q: swap and where?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I had the initial idea that it would be more space economic if I'd add a
> large swap space to Dom0 and do not assign individual DomUs any swap.
> However I had in mind that XNE would also give virtual memory to DomUs as
> RAM (virtually). It seems that Dom0 can only give out physical RAM as
> virtual RAM for DomUs. Is that correct?

Virtual machines under Xen get a real allocation of physical memory, they're 
not paged by either dom0 or Xen itself.  This avoids a few potential issues 
(e.g. "double paging") and makes the implementation simpler.

The argument that goes along with this is that the domUs themselves are best 
placed to understand which bits should be swapped out, so they should be in 
charge of swapping their own stuff.  As a result of this, they all need to 
have swap partitions.

> The other question is: Does something like a "cluster swap partition" exist
> (in a sense of cluster file system, where all DomUs could swap to)?

Nope.  You could use VBDs backed by sparse files to hold the domU swap 
partitions.  That would mean that space wasn't used on the real disk until 
the actual domU filled it with something (although obviously, eventually 
you'd probably end up with every bit of the file being used, making it 
non-sparse and therefore nolonger a space saving).

> And 
> when we are at it: Did anybody manage to share a local filesystem using
> some cluster filesystem between DomUs?

I'm pretty sure this has been done, yes.

Cheers,
Mark

> I'm thinking about being space efficient.
>
> Regards,
> Ulrich
>
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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