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

[Daniel P . Berrangé]

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 ?

>  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.

With regards,
Daniel
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