From mboxrd@z Thu Jan  1 00:00:00 1970
From: James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>
Subject: Re: Device Namespaces
Date: Mon, 30 Sep 2013 08:37:19 -0700
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To: Greg Kroah-Hartman <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>
Cc: Linux Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>, Kay Sievers <kay.sievers-tD+1rO4QERM@public.gmane.org>, Stephane Graber <stgraber-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>, lxc-devel <lxc-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org>, mhw <mhw-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, devel <devel-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
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On Thu, 2013-09-26 at 10:07 -0700, Greg Kroah-Hartman wrote:
> On Thu, Sep 26, 2013 at 08:01:31PM +0300, Janne Karhunen wrote:
> > That being said, our wish would be to support any combination of
> > OS's and frankly, I'd be slightly annoyed to tell the customer that
> > they can't do two Androids or we magically run out of bits.
> 
> If you want to support "any" combination of operating systems, then use
> a hypervisor, that's what they are there for :)

No that's not quite the right way to think about it: The correct
statement is only use a hypervisor if you need different kernels.  With
Windows, it happens to be true that you need a different kernel for each
different OS version.  However; with Linux, thanks to strong ABI
backwards compatibility, you mostly don't.  The way OpenVZ works today
is that it installs a modified kernel which can then bring up every
Linux OS in a separate container.  Our use case is the hosters that give
you root login to a virtual private server and allow you to upgrade it
on your own.  The reason for using a container rather than a hypervisor
is the old density and elasticity one:  3x the density (i.e. 1/3 the
overhead cost to the hoster) and the boot only needs to start at init,
not bring up of virtual hardware and booting a second kernel.

James