Subject: Re: [vserver] Pros and Cons of 32/64bit guests on 64bit host
From: Nicolas Cadou <ncadou@cobi.net>
Date: Fri, 22 Aug 2008 09:58:03 -0400
Fri, 22 Aug 2008 09:58:03 -0400
Le August 22, 2008, Stephen Liu a écrit :
> > With KVM you can overcommit, as with others I believe, ie for you 8GB
> > machine you can have 4 VMs with 4GB each; physical RAM will get used
> > according to each VM's usage.
>
>  Sorry I don't follow.  If 4 VMs (not KVM/QEMU) with 4G each then I need
>  16G RAM at least.

Not necessarily. Even if a VM is configured as a 4GB machine, the full 4GB is 
not always used. Right after boot, for example a Windows VM should consume 
less than 10% of that. The remaining of the RAM is not, then, allocated on 
the host either. That's why the ability to overcommit is interesting. 
Overcommiting means allowing the allocation of more memory than physically 
available.

In the end it all depends on the actual combined memory usage of all the 
programs that run on the host and in the guests.

Here, Linux-VServer shines, being an OS-level virtualisation solution, because 
some of the memory usage can be shared between guests when files are 
hardlinked, allowing to run much more guests than with other, machine-level, 
virtualization techniques.

>  > I would have preferred to run the vservers directly on the host, but
>  > there is
>  > no official Linux-VServer enabled kernel in Ubuntu Hardy, and I don't
>  > need
>  > the maintenance overhead of having to recompile a kernel every now
>  > and then,
>  > so KVM it is for now.
>
>  Is there an item in Ubuntu Hardy kernel for enabling Linux-VServer to
>  rebuilt the kernel?

No, there is not. Not an official one, at least, and I'm not aware of an 
unofficial one either.

-- 

Nicolas Cadou

Cobi Informatique Inc.
Tel: (450) 266-2420 (305)
Fax: (450) 266-2415


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