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

[David Mansfield]

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?

Thanks,
David