Re: [PATCH v2 9/9] docs: Start documenting VM templating

[David Hildenbrand]

On 22.08.23 15:47, Daniel P. Berrangé wrote:
> On Tue, Aug 22, 2023 at 01:44:57PM +0200, David Hildenbrand wrote:
> > Let's add some details about VM templating, focusing on the VM memory
> > configuration only.
> >
> > There is much more to VM templating (VM state? block devices?), but I leave
> > that as future work.
>
> Then there's the supposedly "unique" hardware identifiers, most notably
> VM UUID & NIC MAC addr that don't change if you create many VMs from
> a "template". Or from the guest OS there are "unique" things like
> /etc/machine-id, SSH host keys, web server certificates, etc.
>
> The vmgenid device at least provides a way for guest OS to get notified
> to update its unique resources/identifiers, but doesn't solve the overall
> VM UUID. NIC MAC addr could be solved by hotunplug+plug either side of
> creating the template & instantiating the template.
>
> > Signed-off-by: David Hildenbrand <david@redhat.com>
> > ---
> >   docs/vm-templating.txt | 109 +++++++++++++++++++++++++++++++++++++++++
>
> Can you make this doument RST from the start and link to it from
> somewhere appropriate in our documentation. Perhaps it should live
> under the docs/system/ directory ?

I blindly did what memory-hotplug.txt and nvdimm.txt do. I can make it a 
RST and move under docs/system [+ link it in the index]

> >   1 file changed, 109 insertions(+)
> >   create mode 100644 docs/vm-templating.txt
> >
> > diff --git a/docs/vm-templating.txt b/docs/vm-templating.txt
> > new file mode 100644
> > index 0000000000..419362c1ea
> > --- /dev/null
> > +++ b/docs/vm-templating.txt
> > @@ -0,0 +1,109 @@
> > +QEMU VM templating
> > +==================
> > +
> > +This document explains how to use VM templating in QEMU.
> > +
> > +For now, the focus is on VM memory aspects, and not about how to save and
> > +restore other VM state (i.e., migrate-to-file with 'x-ignore-shared').
> > +
> > +Overview
> > +--------
> > +
> > +With VM templating, a single template VM serves as the starting point for
> > +new VMs. This allows for fast and efficient replication of VMs, resulting
> > +in fast startup times and reduced memory consumption.
> > +
> > +Conceptually, the VM state is frozen, to then be used as a basis for new
> > +VMs. The Copy-On-Write mechanism in the operating systems makes
> > +sure that new VMs are able to read template VM memory; however, any
> > +modifications stay private and don't modify the original template VM or any
> > +other created VM.
>
> I feel like we should have a paragraph at the top here explicitly calling
> out the dangers of templating, wrt to unique data in the hardware and guest
> OS. Don't have to provide solutions, just more of a scarcy "here be dragons"
> warning to users who might be tempted to try this.

Agreed, I'll use some of your information above, thanks!

--
Cheers,

David / dhildenb