Re: Shadow page tables?

[Peri Hankey <mpah@thegreen.co.uk>]

Hello

Some time ago you talked about copy-on-write memory to enable large 
numbers of nearly identical machines to run on the same physical 
hardware. I have also thought about that - after all equivalent  
mechansisms already exist to provide copy-on-write semantics for process 
forking (and also I think in the /proc/mm mechanisms used in some 
versions of UML separate kernel address space (SKAS) handling).

Have you advanced along this path, or is anyone actively doing that? The 
problem you found the other day suggests that you were looking pretty 
closely at code that relates to this question. My very crude 
understanding is that one would want to share the physical memory 
allocation that is currently given to a single xenU domain between a 
group of xenU domains each of which has its own  copy-on-write mapping 
that starts from a read-only image that is shared between all of them. 
The table grant mehanism would need to understand the relationship 
between members of such a group of domains. But as I say, this is still 
very hazy to me.

On the other point you raised, about copy-on-write filesystem images, I 
looked at evms2 (http://evms.sourceforge.net/), which sits above lvm2 
and other systems and offers its own snapshot feature. It is only usable 
on disks that it controls, so you either have to use a separate disk 
from the one you boot from, or you have to use an initrd to be able to 
boot from an evms volume. The snapshot feature seemed 'snappier' (I was 
able to create a number of snapshot objects and activate them without 
memory problems of the kind that I encountered with lvm2 snapshots).

However it didn't seem possible to make it provide multiple persistent 
writable snapshots of the kind that would support multiple xenU domains 
where each uses its own copy-on-write image of an underlying readonly 
root filesystem. Did I miss something? Has anyone else tried using evms 
for these purposes?

I saw Ian's comments about gnbd and csnap 
(http://sources.redhat.com/cluster/), but it seems that the csnap 
mechanism is in a pretty early state of development.

Regards
Peri

Michael Vrable wrote:


Michael Vrable wrote:

>On Fri, Oct 08, 2004 at 10:11:18AM +0100, Keir Fraser wrote:
>  
>
>>We intend to flesh out the shadow p.t. code a little mroe to support
>>full memory virtualisation. It's not there yet, but it won't require
>>an enormous amount of code to get it going.
>>    
>>
>
>Any idea what the time frame for this is?  I was considering taking a
>stab at it myself, but don't want to duplicate work.
>
>My longer term goal is to try to get copy-on-write sharing of memory
>pages between domains and to see how far Xen can scale in running many
>nearly-identical virtual machines.  To implement this, however, will
>require (at least as I've thought it through) memory virtualization.  So
>that was the first thing I was planning to work on.
>
>If no one else was actively working on this, I'm happy to discuss the
>design and contribute what I end up with.  (Both for the memory
>virtualization and the copy-on-write work.)
>
>(This is also the reason for my interest in copy-on-write disks, though
>it's looking like LVM may be too heavyweight for supporting large
>numbers of VMs.)
>
>--Michael Vrable
>
>
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