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

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