Empty cdrom drive.

[Ayal Baron]

----- Original Message -----
> On Wed, Feb 15, 2012 at 07:16:15AM -0500, Igor Lvovsky wrote:
> > 
> > 
> > > -----Original Message-----
> > > From: Dan Kenigsberg [mailto:danken at redhat.com]
> > > Sent: Wednesday, February 15, 2012 1:25 PM
> > > To: Igor Lvovsky
> > > Cc: Livnat Peer; Doron Fediuck; Ayal Baron;
> > > ovirt-devel at redhat.com
> > > Subject: Re: Empty cdrom drive.
> > > 
> > > 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.
> > > 
> > 
> > -1. I am disagree with this approach.
> > If Engine can send hackAutoCD for old VMs I can't find a reason why
> > do not
> > send device instead.
> 
> My suggestion was that Engine would send hackAutoCD for *ALL* VMs -
> until Engine is capable to handle VMs without CD devices.

Absolutely not.  The semantics are funky and API changes are much harder to do then simple code change.  it's a hack, don't put it in the API!

> 
> Vdsm would ignore this flag if a cdrom is already specified in
> "devices". Thus, future Engines can drop the hackAutoCD flag and gain
> cdrom-less VMs.
> 
> > As you mentioned above vdsm anyway will return this cdrom device to
> > Engine
> > and engine will
> > need to put it in DB. This will cover all old VMs and there is no
> > reason
> > do not send empty cdrom device
> > for new created VM.
> > 
> > > 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.
>