Ubuntu Servers :: Df -h Not Updating With Changes Made By Other Machine?

I'm running a virtualised backupserver. This makes backups to an LVM volume on the host. /dev/mapper/system-backupserver is put through as a block device to the KVM-based, libvirt-managed backupserver and there it's mounted (obviously) rw.

I've also mounted the same volume, as ro, on the host, so as to get at my backups more easily. So now the same partition is mounted on two different machines, both thinking they've got sole access.

Now when the backupserver VM alters the contents of this partition, it sees this happening normally on its df report, but on the host machine, this remains mostly unseen. I tried doing a mount -o remount, but that doesn't help, although umount/mount will do it.

Meanwhile I seem to be able to get at all the data just fine, and it's just the total used/free that gets out of whack. Is there any way to regenerate/reread those totals without doing a full umount/mount cycle?