On 12/19/2014 04:23 PM, Boylan, Ross wrote: > Hi. I have some narrow questions and a larger one. [please configure your mailer to wrap long lines] > > If I make an existinig LVM VG a storage pool, can I use some of the LV's for libvirt and some for the host? Or does the VG needed to be completely dedicated to virtualization? It is technically possible to share VGs, but I would probably advise against it. Why? Unless you set up udev rules or other tricks to ensure that the host does not probe the LVs dedicated to guest storage, you could risk accidentally mounting the LV in the host at the same time it is passed through to the guest, and that is a potential recipe for data corruption. It's easier to isolate resources if you can guarantee that the storage pools being allocated to guests are distinct from the storage pool that the host uses for itself. > > Assuming mixed use is possible, is it possible to do an LVM snapshot of an LV in use for a VM? The snapshot would have to be initiated and used on the host, I assume. LVM snapshots are definitely possible for offline guests. Also, with new enough libvirt, you can freeze guest I/O, take an LVM snapshot, then resume guest I/O; although I haven't played with that much. However, any scenario that requires changing the file name opened by qemu to represent the latest data is not very well supported (qemu does not yet have a way to pivot over to a new external file name that represents the same contents as the pre-snapshot state). On the other hand, qemu is definitely gaining some cool features like external snapshot (where the original file becomes a read-only backing file to a temporary qcow2 file), then you do external backup of the original file, then do an active commit back into the original, which can help in getting clean scenarios for useful backups via LVM snapshots or other means without risking data corruption. > > How dynamic is this? If I create a new LV, is it possible to see that immediately from, e.g., virt-manager and add it as a virtual disk to a running VM? If I resize the LV, will the VM know so that, e.g., I could do an online resize of the filesystem? Refreshing the pool view will see new LVs, and there are libvirt APIs for resizing a VM (you DO have to use the libvirt API; you can't just blindly assume that the guest will see more storage unless you have used the right APIs). > > My larger question is what the recommendations are for managing disks or LV's with libvirt and KVM on Linux. In particular, I'd like to be able to grow storage space as the the VM needs grow, and snapshot (in the LVM sense) some of the block devices behind filesystems for backup. Although it would be nice to grow space while the VM's are running, it's not a requirement. Sounds like you are interested in reinventing what VDSM already does (VDSM uses LVM volumes to grow storage as needed for guests across a cluster). > > Following advice on the KVM list, I have been using raw LV's as virtual disks for the VM's. The host is currently running with a large, encrypted LVM VG. It would be convenient to use it for the VM's. As for whether files in the host filesystem atop its LVM, vs. separate LVMs entirely, is more efficient, you'd have to benchmark the difference it makes (fewer layers by using LVMs directly probably makes for more efficiency, but it also means more effort in setting things up compared to just using files in the same host filesystem). -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org _______________________________________________ libvirt-users mailing list libvirt-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvirt-users
