Is there an atomic way to get the next free VM_ID and reserve it?

[Cylindric]

Hi all. I'm using automation to deploy VMs onto PVE, and I've come across a problem. In order to create a VM, I think I have to provide a VMID, for example in the API or with `qm create`. I can get the next free ID with `/cluster/nextid`, however if two processes try to build a VM at the same time, they'll fail if they both check `nextid` at the same time before they create a VM.
I need a way that's similar to how IPAM does "next free IP address" - it needs to reserve the one it returns so two things don't get the same number, or I need a way of call create that doesn't need a VMID.

 

[LnxBil]

Why do you need the next free ID? Why don't you just pick one at random and check if it's free? You will - of course - still have a non-zero probability of a race condition, but it may occur a lot less frequent.

 

[Cylindric]

Well, because asking if one exists and using it is better than assuming it doesn't and cobbling something together that isn't necessary.

 

[fweber]

Hi, there is currently no way to atomically lookup and reserve the next free VMID. See [1] for some related discussions. For your automation usecase, the best course of action would be the one suggested by LnxBil: Just generate a random VMID (e.g. between 100 and 1_000_000) and try to create a VM with that VMID using the POST /nodes/{node}/qemu [2] endpoint. Note that this request will fail if the VMID is already taken -- in that case, the caller should just generate a new random VMID and try again. With this, it should be impossible to run into a situation where two callers assume they "own" the same VMID.

[1] https://bugzilla.proxmox.com/show_bug.cgi?id=4369
[2] https://pve.proxmox.com/pve-docs/api-viewer/index.html#/nodes/{node}/qemu