[Libvir] Tricky provisioning problem with inactive domains

[Hugh Brock]

Daniel P. Berrange wrote:
> Adding support for inactive domains was supposed to make everyone's life 
> easier, but as luck would have it, its actually made one thing very much
> harder. In the virt-inst/virt-manager tools provisioning works something
> like this:
> 
> In paravirt case:
> 
>  - Create a guest using an explicit kernel/initrd from the images/xen
>    directory on the install CD
>  - Write a config file to /etc/xen setup to boot using pygrub
> 
> In fullvirt case:
> 
>  - Create a guest booting directly off a CDROM
>  - Write a config file to /etc/xen setup to boot off the harddisk
> 
> So in both these cases, the libvirt XML config for the very first boot of
> the guest is different, from the XML config for subsequent  boots. With 
> the new inactive domain support in libvirt & xend, we can't write out 
> config files directly, instead there is the virDomainDefine() API, which
> calls to appropriate APIs in XenD. And this is where the problem arises:
> 
>  1. If we call virDomainDefine() to write the long term config, then 
>     virDomainStart() will not be using the correct boot method for the
>     initial install.
>  2. If we call virDomainDefine() to write the initial install config,
>     then virDomainStart() will kick off the install correctly, but on
>     subsequent boots we'll end up booting the installer again instead
>     of the just installed OS.
>  3. We could just use virDomainCreate() to start installer, and try to
>     use virDomainDefine() to write the long term config - the latter
>     call will fail though because there will already be a running guest
>     with that name.
>  4. Conversely if we use virDomainDefine() to write the config, and then
>     tried to create a one-off guest with virDomainCreate() the latter
>     will fail due to duplicate names.
> 
> So, thus far the only way out of the trap I can think of is:
> 
>  1. Use virDomainCreate() to kick off the initial install
>  2. Poll virDomainLookupByXXX() to watch for this initial guest shutting
>     down
>  3. Write out the persistent config using virDomainDefine()
> 
> The big problem with this, is that if the user were to exit virt-manager
> sometime after the guest install starts, but before step 3, the config
> for the guest will never be written, even though it has successfully
> installed.
> 
> There are two further ideas I've had - both requiring additional APIs in
> libvirt & probably XenD too.
> 
>  - Make it possible change the boot configuration of an existing guest.
>    This would let us do:
> 
>       1. Use virDomainDefine() to define a config file suitable for installing
>          the guest, ie using explicit kernel/initrd
>       2. Use virDomainStart() to kick off the installer
>       3. Uew new  API to change the guest config to remove explicit kernel
>          & initrd config, and add a bootloader for pygrub. Or in HVM case,
>          switch boot order to use harddisk instead of CDROM & detach the
>          CDROM device.
> 
>  - Make it possible to start an existing inactive guest using an alternative
>    one-off configuration. This would let us do:
> 
>        1. Use virDomainDefine() to define a config file suitable for running
>           the guest during normal operation.
>        2. Use virDomainStartConfig(xml) to start the guest with a special
>           config suitable for installing the guest.
> 
> 
> Ultimately I think we do need the means to change arbitrary parts of a guest
> configuration, so I think the first option would be the preferred approach.
> The trouble is that I think implementing this would require using the new 
> XenAPI, or adding a number of new methods to the legacy SXPR API which is not
> really very desirable.
> 
I rather like the idea of having both features available, actually. The 
ability to start a guest with an alternate one-off config (not just a 
different kernel and initrd, but perhaps different memory, disk, network 
configs?) strikes me as useful, especially in automated test harness 
applications. And as you say the ability to programmatically modify the 
config is also useful.

--Hugh