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Re: [Libvir] Tricky provisioning problem with inactive domains
- From: Karel Zak <kzak redhat com>
- To: Daniel Veillard <veillard redhat com>
- Cc: libvir-list redhat com
- Subject: Re: [Libvir] Tricky provisioning problem with inactive domains
- Date: Wed, 17 Jan 2007 11:03:31 +0100
On Mon, Jan 15, 2007 at 09:46:37AM -0500, Daniel Veillard wrote:
> On Mon, Jan 15, 2007 at 02:14:36PM +0000, 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.
> Well the core of the problem is that there is 2 set of information, distinct
> in their value and meaning and we have only one placeholder for both. It seems
> to me that basically we are overloading the <os> block while we would need
> a separate <install> block containing the informations needed to install the
> Why not plit them out, and keep both in the configuration ? It seems it would
> open the door to:
> - the user option of restarting the installation of a domain, which
> with virtualization is certainly a more common operation than
> reinstalling a bare system
> - allow to keep track of how and where a given domain was installed
> and with what data
> - ease provisioning of a similar domain based on the informations
> provided in that <install> block
Yes. The idea with <install> and <os> is ***very good***.
> Basically we could create the domain configuration with both the <os>
> and <install> block, the install one for example missing an installation
> date, suggesting the user to run the installation process instead of
> the normal boot of the instance.
> I'm not even sure libvirt would have to be modified for this, it will by
> default ignore the <install> block, and the application on top of it could
I think libvirt would have to be modified for this -- at least a
separate API which is able to loads data from the <install> block and
returns it to libvirt based application. I'd like to use
virt-install --config /etc/xen/myvm.xml
when I need to re-install the domain.
> just modify the XML before calling Create() . Making libvirt install aware
> would force an API extenstion though, I'm not 100% sure that's really needed.
It's cheap add there this API extenstion for <install> and it's
without an impact to the old libvirt API. Our life will be more funny
with this API ;-)
> Did I understood correctly the problem ? I think adding extra informations
> about the install data is in any case a good idea, especialy when one think
> in terms of provisionning.
Karel Zak <kzak redhat com>
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