It's not so unusual. It just wasn't easy to come across before widespread virtualization.
Nesting LVM setups like this CAN be done, but generally shouldn't.
Essentially, LVM works by detecting the physical volumes that are part of the LVM. Since you are setting up an LVM that is effectively on another LVM device, the detection can get weird--especially if you name an inner LVM volume group the same name as an external LVM volume group. Let's just say that mixing LVM data from the inner and outer devices can happen, and then things can get really broken.
The easiest way to do this (and it's not so easy) is to set up LVM to scan your other LVM devices using a second lvm.conf. Essentially you configure both copies of LVM (the internal one and the external one) in separate files with careful limits on which devices may be scanned for PVs.
A generally better solution is just to use LVM on the outside and pass through the LVs to look like local disks. The downside is that it requires cooperation from the Dom0 to resize and snapshot; and that FS extensions currently require rebooting the VM to pick up the size changes. The upside is the LVM is really simple to manage if you just do it outside, and the volume is available to the Dom0 for mounting like you described.
Unless you absolutely need snapshotting normally inside or live resizing, I wouldn't go through the trouble of running LVM inside of LVM. It is fraught with peril.
I am aware that you are going to have to reinstall your systems, but, if they are doing anything important, you should avoid things that can fatally confuse LVM (like nested LVM setups). While you're at it, don't partition the disks inside of the DomUs either. Just pass through the partitions directly.
Good luck. On Mar 30, 2008, at 4:34 PM, Simone wrote: I guess I am trying to do something unusual :)
Any thoughts?
Simone
On Fri, Mar 28, 2008 at 6:47 PM, Simone < dezmodue@xxxxxxxxx> wrote: Hi list,
I am experiencing with xen on centos5 and so far everything is going well, I am really pleased. The domU have a dedicated LV (/dev/vg1/xenVM1, /dev/vg1/xenVM2 etc) and the guest OS is Centos4. At guest install time I have choosen to use LVM so that also inside the guest I have /dev/vg0/root, /dev/vg0/tmp etc. Is there a way to mount and edit the guest filesystem to customize files etc?
The idea would be to have a guest template that can be cloned and then edited to generate new VMs.
Thanks, have a good weekend
Simone
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