proxmox / alter default create vm parameters

Written by
Walter Doekes
Published on 2021-04-07

The Proxmox Virtual Environment has defaults when creating a new VM, but it has no option to change those defaults. Here's a quick example of hacking in some defaults.

Why? (Changing SCSI controller does not change existing disks)

In the next post I wanted to talk about /dev/disk/by-id and why disks that use the VirtIO SCSI controller do not show up there. A confusing matter in this situation was that creating a VM disk using a different SCSI controller and then switching does not change the storage driver for the existing disks completely!

If you're on Proxmox VE 6.x (observed with 6.1 and 6.3) and you create a VM with the VirtIO SCSI controller, your virtual machine parameters may look like this, and you get a /dev/vda device inside your QEMU VM:

/usr/bin/kvm \
  ...
  -device virtio-blk-pci,drive=drive-virtio0,id=virtio0,bus=pci.0,addr=0xa \
  -drive file=/dev/zvol/somedisk/vm-NNN-disk-0,if=none,id=drive-virtio0,format=raw

But if you create it with the (default) LSI 53C895A SCSI controller first, and then switch to VirtIO SCSI, you still keep the (ATA) /dev/sda block device name. The VM is started with these command line arguments:

/usr/bin/kvm \
  ...
  -device virtio-scsi-pci,id=scsihw0,bus=pci.0,addr=0x5 \
  -drive file=/dev/zvol/somedisk/vm-NNN-disk-0,if=none,id=drive-scsi0,format=raw \
  -device scsi-hd,bus=scsihw0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0,id=scsi0

If you look at the configuration in /etc/pve/qemu-server/NNN.conf, both would have:

scsihw: virtio-scsi-pci

But the disk configuration type/name is different:

virtio0: somedisk:vm-NNN-disk-0,size=32G

vs.

scsi0: somedisk:vm-NNN-disk-0,size=32G

This virtio-scsi-pci + scsi0 is turned into -device virtio-scsi-pci flags.
While virtio-scsi-pci + virtio0 translates to -device virtio-blk-pci.

It's not a bad thing though, that it does not change from scsi0 to virtio0. After all, if the device did change from /dev/sda to /dev/vda, your boot procedure and mounts might be impacted. But it does mean that you want the VirtIO SCSI option selected before you create any disks.

How? (Hacking defaults into pvemanagerlib.js)

In the pve-manager package, there's a /usr/share/pve-manager/js/pvemanagerlib.js that controls much of the user interface. Altering the default appears to be a matter of:

--- /usr/share/pve-manager/js/pvemanagerlib.js
+++ /usr/share/pve-manager/js/pvemanagerlib.js
@@ -21771,7 +21771,7 @@ Ext.define('PVE.qemu.OSDefaults', {
         scsi: 2,
         virtio: 1
       },
-      scsihw: ''
+      scsihw: 'virtio-scsi-pci'
     };

     // virtio-net is in kernel since 2.6.25

For bonus points, we can disable the firewall default, which we manage elsewhere anyway:

--- /usr/share/pve-manager/js/pvemanagerlib.js
+++ /usr/share/pve-manager/js/pvemanagerlib.js
@@ -22434,7 +22434,7 @@ Ext.define('PVE.qemu.NetworkInputPanel',
         xtype: 'proxmoxcheckbox',
         fieldLabel: gettext('Firewall'),
         name: 'firewall',
-        checked: (me.insideWizard || me.isCreate)
+        checked: false
       }
     ];

@@ -27909,7 +27909,7 @@ Ext.define('PVE.lxc.NetworkInputPanel',
       cdata.name = 'eth0';
       me.dataCache = {};
     }
-    cdata.firewall = (me.insideWizard || me.isCreate);
+    cdata.firewall = false;

     if (!me.dataCache) {
       throw "no dataCache specified";

Of course these changes will get wiped whenever you update Proxmox VE. Keeping your hacks active will be an exercise for the reader.

proxmox / alter default create vm parameters