[Libguestfs] IO rate of tar-in, what can we expect on a qcow2 image?

Hi all, We have done several speed tests on a qcow2 Linux Image to test how fast tar-in with a big tarball can be. Virtio seems to be active, and we get transfers in a range from 100-160MB/sec, independent of the disk speed on the host. For example we had a 20 core host system with 900MB for serial writing and 350MB for mixed read/write on the native filesystem. We've expected a faster tar-in for qcow2 there than on a small test system with some slow disks and only two cores. But the rates where nearly the same. We tried really hard to do some optimizing and using big files inside the tar (for emulating serial writing), but we managed only to get those 160MB/sec, but not more. The IO-rate for serial writing inside a running VM-Image on the same qcow2 image is much more higher, dependent of course on the underlying disks of the host. Here we get clear benefits of fast storage. Has anybody of you got more than those 160MB/sec with tar-in, and if so, how did you do it? Are there maybe some limitations in the XDR/RPC mechanisms of tar-in ? Best regards: Martin Blapp