Re: [Libguestfs] [PATCH virt-v2v v2] v2v: -o libvirt: Remove cache=none.

Traditionally if you did live migration (KVM to KVM), you had to ensure that cache=none was set on all disks of the guest up front. This was because of quirks in how NFS works (I think the close-to-open consistency and the fact that during live migration both qemus have the file open), and we had to assume the worst case that a guest might be backed by NFS. Because of this when virt-v2v converts a guest to run on KVM using libvirt it sets cache=none. This is not necessary with modern qemu. If qemu supports the drop-cache property of the file block driver, which libvirt will automatically detect for us, then libvirt live migration is able to tell qemu to drop cached data at the right time even if the backing is NFS. It also had a significant performance impact. In some synthetic benchmarks it could show 2 or 3 times slower performance. Thanks: Ming Xie, Peter Krempa. ---