libvirt users - Dec 2011 - for a guest accessing host "full disk", how to prevent host vgscan

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.

Oh... I'm using centos 6.2 on host and guest, 
libvirt-0.9.4-23.el6_2.1.x86_64, although it hardly seems a libvirt 
issue at all, it seems to be a lvm host issue mainly.

Thanks,
David Mansfield

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.

Regards,
Daniel
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