[ovirt-devel] Empty cdrom drive.

[Dan Kenigsberg]

On Tue, Feb 14, 2012 at 10:59:22AM -0500, Igor Lvovsky wrote:
> 
>   Hi,
> I want to discuss $subject on the email just to be sure that we all on the
> same page.
> 
> So, today in 3.0 vdsm has two ways to create VM with cdrom :
>  1. If RHEV-M ask to create VM with cdrom, vdsm just create it
>  2. RHEV-M doesn't ask to create VM with cdrom, vdsm still creates VM with
>     empty cdrom. Vdsm creates this device as 'hdc' (IDE device, index 2),
>     because of libvirt restrictions.
>     In this case RHEV-M will be able to "insert" cdrom on the fly with
>     changeCD request.
> 
> In the new style API we want to get rid from stupid scenario #2, because 
> we want to be able to create VM without cdrom at all.

> It means, that now we need to change a little our scenarios:
>  1. If RHEV-M ask to create VM with cdrom, vdsm just create it
>  2. RHEV-M doesn't want to create VM with cdrom, but it want to be able to
>     "insert" cdrom on the fly after this. Here we have two options:
>     a. RHEV-M should to pass empty cdrom device on VM creation and use
>        regular changeCD after that
>     b. RHEV-M can create VM without cdrom and add cdrom later through
>        hotplugDisk command.

Let's leave hotpluggin for a later discussion. Currently I am worried
about backward and forward compatibility.

1. Currently, all VMs created by ovirt-Engine has an IDE cdrom device. This
behavior should be maintained when Engine is upgraded, to minimize
surprises to guests.

2. In the new "devices" API introduced by Igor, Engine is responsible to
know about all guest devices and their addresses.

1+2=3. Engine has to be aware of the fact that even if it did not
explicitly request for a cdrom, such a device exist.

4. Vdsm would very much prefer that Engine explictly request that an
empty cdrom device is included. This would allow us to start VMs with no
cdrom device at all in the future.

I understand that this may be a complex feat for Engine, as it requires
a complex upgrade path to the VM data base. To be done correctly, it
requires a compatible change to the ovirt API, too.

5. I suggest a hackish API that would let us solve the problem in
stages: Engine would not have to explicitly list an empty CD. However,
it would send a hack flag: hackAutoCD=True for all VM starting up.

If this flag is True, Vdsm would add an IDE CDROM to the devices list.

In the future, Engine would drop this flag and specify the CDROM only
when needed.

Please note that (3) is still correct - Engine would see the CDROM
device and its address even if it was empty when the VM started running.


Comments?

Dan.