On 12/19/2014 04:23 PM, Boylan, Ross wrote:> Hi. I have some narrow questions and a larger one.
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>
> 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