Re: [Xen-users] use existing lvm volume as root in hvm/pv guest

[Sean Greenslade <sean@xxxxxxxxxxxxxxxxxx>]

Going to try my best to trim this down while keeping it readable.

On Fri, Jun 07, 2013 at 12:24:14AM +0200, daniel boone wrote:
> On Thu, Jun 6, 2013 at 3:41 AM, Sean Greenslade 
> <sean@xxxxxxxxxxxxxxxxxx>wrote:
> 
> > On Wed, Jun 05, 2013 at 11:55:44PM +0200, Daniël Boone wrote:
> > > I want to accomplish the following:
> > >
> > > I want to have a xen guest that I can also boot physically.
> > > Some indepth explanation:
> > >
<snip>
> >
> To clarify:
> 4x 2 TB in raid10 with each disk that has 3 partitions:
> p1: bios_grub partition
> p2: swap
> p3: raid partition.
> 
> The raid10 spans all p3 partitions from the disks.
> On top of this I have LVM with 1 pv , 1 vg and 3 lv's(root, data, vm_data)
> 

Perhaps I am still misunderstanding. Are all four disks identically
partitioned? Because from what I understand of this, you have four
bios_grub partitions and four swap partitons, and then four RAID
partitions that get RAID10 software assembled into the single PV for
an LVM VG.

>  > I have tried the following scenario's without success.
> > >
> > > Experiment 1
> > > Ubuntu 13.04 with xen 4.2.2
> > > create hvm guest with passthrough of intel sata controller: can see
> > > the disks, but cannot boot from it.
> > > I tried to complie xen myself but failed.
> > > raid and lvm managed by guest
> > >
> > > Experiment 2
> > > Ubuntu 12.04 with xen 4.1.0
> > > create hvm guest with passthrough of intel sata controller: can see
> > > the disks, but cannot boot from it.
> > > raid and lvm  managed by guest
> > >
> >
> > What are the errors? 
<snip>
> If you boot a rescuecd it can see the disks and raid. I this this has to do
> with the bios and qemu version that comes with mainstream xen.

So again, what's the problem? If it can see the disks and assemble the
RAID, there's no reason it shouldn't be able to boot. What exactly is
preventing it from booting? Specific error messages well help.

> 
> > > Experiment 3
> > > Ubuntu 12.04 with compiled xen 4.2.2  and qemu-upstream and seabios 1.7.2
> > > create hvm guest with passthrough of intel sata controller: can boot
> > > from the disks if  device_model_version = 'qemu-xen' and
> > > device_model_override = '/usr/lib/xen/bin/qemu-system-i386' is used
> > > in config. But raid is not stable.
> > > raid managed by guest
> > >
> >
> > What do you mean by "RAID is not stable"?
> >
> > When booting, disks are dropping out, getting messages that the response
> of the disk is to slow.
> 

Once again, specific error messages would be helpful, though I suspect I
will have little to offer in that regard. It seems like that would be
from a QEMU issue.

> 
> >  > Experiment 4
> > >
> > > Ubuntu 12.04 with compiled xen 4.2.2  and qemu-upstream and seabios 1.7.2
> > > create hvm guest with passthrough physical disks seperately : the
> > > raid is detected in dom0 so this also poses an issue. I tried
> > > booting raid=noautodetect as grub parameter  but without success,
> > > raid still starts.
> > > raid and lvm managed by guest
> > >
> >
> > This seems more like an Ubuntu issue. There is most definitely a way to
> > prevent the dom0 from assembling the array. I would suggest asking an
> > Ubuntu mailing list about that, as I have no experience with Ubuntu.
> >
> I think it's more a kernel/initrd issue. I know the parameter is
> raid=noautodetect is the correct one.
> I can't remove the modules from the initrd as I can't find a way to unpack
> the xen initrd.

If you are truly setting that kernel parameter correctly (and I would
suggest confirming that by editing the selected boot line in Grub as it
boots), then you have more than likely discovered a kernel bug. That may
warrant further investigation. 

Also, there is no Xen initrd. The initrd is the dom0's initrd, which is
why I said this is more of an Ubuntu issue than a Xen issue. You want
Ubuntu to not assemble the RAID on boot, something that Xen should not
be affecting in any way.
> 
> > > Experiment 5:
> > >
> > > This is where I'm lost
> > > Ubuntu 12.04 with compiled xen 4.2.2 and no qemu-upstream as I
> > > cannot boot from cdrom with qemu-upstream
> > > I was thinking of adding a small file based disk for /boot and grub
> > > and  using the previously installed lvm volume as root.
> > > When I boot the guest from a rescuecd no diskpartitions are detected
> > > which is logical as the lvm volume is used as a filesystem and not a
> > > raw lvm disk.
> > >
> > > Is there a way to use the filebased lvm as root?
> > > If I use a lvm volume as a raw disk, would it be possible to boot
> > > from it with grub without virtualisation.
> > >
> >
> > LVM is always a tricky beast. 
< LVM explanation snipped >
> The filebased boot disk would be a normal disk without any lvm on it.
> I will try if a pv host and see if that will work.

Yes, this is the most sensible route. No sense in making that
complicated. And yes, PV is the preferable way to run domUs. As long as
your domU kenel has the PV drivers compiled in, running it in PV mode
should work fine. You may have to make some strage entries in your
fstab, but it should be doable.

--Sean

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