qm template: what does it _do_? what makes a template different from a (k)vm?

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johnnyutahh

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`qm template`: what does it _do_? what makes a template different from a (k)vm?

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I see references to how running `qm template [vm_number]` creates a template that makes future (linked or full) cloning better, more-efficient.

But... what does `qm template` do to the (k)vm? (A bit of web searching did not yield any answers here.) If my team understands better even just a little bit on what's happening "underneath the hood" we might make better decisions with our template creation, management, etc.
 
One can convert a VM into a Template. Such templates are read-only, and you can use them to create linked
clones.

It is helpful to convert the VM into a template. From this template you can then quickly create
linked clones. The deployment from VM templates is much faster than creating a full clone (copy).

LearnLInuxTv and ServeTheHome have some additional info on youtube.
 
@chintzy_butters - thanks for your timely replies. I'm not trying to learn (in this thread) the external benefits of creating a template; there's many, many reference for that info. We need not repeat it here. As such, if I'm reading it correctly, none of the above info tells me what the `qm template` command does to a (k)vm to turn it into a template, other than make the template read-only. Is making the vm read-only all that `qm template` does? (I'm guessing you're unfamiliar else you might have stated already - and that's really, totally fine.)

I suspect there's more. eg: does `qm template` strip out unneeded things in the vm to make clones faster? Does it convert the vm to (perhaps) a simple tarball (which is vaguely what I recall an lxc template does)? What else might the `qm template` command do the vm files/disks/etc?

I'm seeking more specifics. I have a deep, experienced, computer-science background. I can take whatever details anybody can give me. To anyone who has knowledge in this arena: pls, do not be bashful, and pls bring on the info.

Best regards,
~J

ps. I had already read https://pve.proxmox.com/wiki/VM_Templates_and_Clones and watched LearnLInuxTv video on cloud-init-template creation before starting this thread; I might dig through ServeTheHome videos later (good tip on that, thanks), but do I really need to spend lots of time finding and watching a bunch of videos to just to learn these details?
 
Last edited:
https://git.proxmox.com/?p=qemu-ser...bc52e2cce843407bf3;hb=refs/heads/master#l4168

is the API endpoint, does some checks, then calls

https://git.proxmox.com/?p=qemu-ser...1391b3954b122bc416;hb=refs/heads/master#l6557

which converts each disk that is a proper volume managed by PVE into a base volume via

https://git.proxmox.com/?p=pve-stor...e4a8b428c2aebedb9ca;hb=refs/heads/master#l761

which is handled by the plugin for the storage that volume is using:

https://git.proxmox.com/?p=pve-stor...cf878fbed75cd6821d400442;hb=refs/heads/master

finally the API endpoint sets a flag on the VM config to indicate it's a template (this flag is then checked for various other operations, e.g. preventing starting the VM, etc.pp.).

what happens on the storage level depends on the storage, e.g. some file-based storages will set the 'immutable' attribute of the image file, Ceph will rename, create a snapshot, and set the 'protected' flag on that snapshot. most plugins will rename the volume and change the volume ID accordingly to reflect it is a base volume now.
 
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Great details @fabian - thank you!

@dietmar - typically a directive like "Simply compare the config file before and after" means (for me) to run a diff(1) or cmp(1) on config file(s). Can you share where I can find the config file(s) for a (k)vm/template in a zfs-based install? I can then, per your suggest, confirm things for myself.

dietmar said:
fabian said:
https://git.proxmox.com/?p=qemu-ser...bc52e2cce843407bf3;hb=refs/heads/master#l4168

is the API endpoint, does some checks, then calls [...]

finally the API endpoint sets a flag on the VM config to indicate it's a template (this flag is then checked for various other operations, e.g. preventing starting the VM, etc.pp.).

what happens on the storage level depends on the storage, e.g. some file-based storages will set the 'immutable' attribute of the image file, Ceph will rename, create a snapshot, and set the 'protected' flag on that snapshot. most plugins will rename the volume and change the volume ID accordingly to reflect it is a base volume now.
Click to expand...
Basically yes. Simply compare the config file before and after - this will show you the changes.
Click to expand...
 
config files for VMs are in /etc/pve/nodes/NODE/qemu/VMID.conf (parts in caps to be replaced ;)). the current config can also be printed by 'qm config VMID' (the config file might contain snapshots as well, these are not displayed by this command).
 
Ahhh, yes, super-duper helpful (again), thanks @fabian (!).

fabian said:
config files for VMs are in /etc/pve/nodes/NODE/qemu/VMID.conf (parts in caps to be replaced ;)). the current config can also be printed by 'qm config VMID' (the config file might contain snapshots as well, these are not displayed by this command).
Click to expand...
 
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