[libvirt-users] for a guest accessing host "full disk", how to prevent host vgscan

[Daniel P. Berrange]

On Thu, Dec 22, 2011 at 09:20:01AM -0500, David Mansfield wrote:
> 
> 
> On 12/21/2011 05:41 PM, Daniel P. Berrange wrote:
> >On Wed, Dec 21, 2011 at 05:23:33PM -0500, David Mansfield wrote:
> >>Hi All.
> >>
> >>I have a dell system with a H700 raid.  Within the hardware RAID
> >>config I've created a "virtual disk" which I have assigned to one of
> >>my guests.  On the host the device is "/dev/sdb", on the guest it's
> >>"/dev/vdb".
> >>
> >>This works fine.
> >>
> >>Within the guest, we have created lvm PV on /dev/vdb (using the
> >>whole disk - no partitions) and created a volume group.  The guest's
> >>hostname is "argo" and the vg is called "vg_argo_bkup".
> >>
> >>When I reboot the host, it does a vgscan and finds the volume group
> >>and activates it in the _host_, which I need to prevent (I think??).
> >>
> >>I have successfully done this by filtering "/dev/sdb" in
> >>/etc/lvm/lvm.conf (which does NOT work as advertised BTW), but
> >>referencing the extremely volatile SCSI "sd*" names seems a terrible
> >>way to do this.  If I fiddle around in the HW raid config, the
> >>/dev/sd? may change.
> >>
> >>I plan on creating about 10 more VM's spread over a number of
> >>machines over the next weeks with a very similar setup, and the
> >>admin overhead seems like it'll be onerous and error-prone.
> >>
> >>I'd love to be able to filter the volume groups by VG name instead
> >>of pv device node.  The host's hostname is "narnia" and I'd love to
> >>say, 'vgscan --include-regex "vg_narnia.*"' or something similar, if
> >>you get my drift.
> >>
> >>Does anyone have a best practice for this?  I'm sure iSCSI
> >>enthusiasts must have the exact same issue all the time.
> >The recommended approach is not to assign the entire disk to the
> >guest. Partition the host disk, to contain 1 single partition
> >consuming all space, then assign the partition to the guest. Worst
> >case is you loose a few KB of space due to partition alignment, but
> >this is a small price to pay to avoid the LVM problems you describe
> >all to well.
> I don't really understand.  The host still scans the partitions,
> right?  And the partition "dev" names change dynamically if the
> whole-disk changes it's "dev" name.  Won't I still have to list
> specific volatile names in the /etc/lvm/lvm.conf on the host?

The host will see '/dev/sda' and '/dev/sda1', you'll assign
' /dev/sda1' to the guest, and it will appear as /dev/vda.
In the guest you'll create '/dev/vda1' and format it as the
PV. So while the host will see /dev/sda1, it won't see the
nested partition table, and thus won't see the PV

Regards,
Daniel
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