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Could you elaborate on 9p/virtio performance? I've had pretty good results booting qemu/kvm VMs with 9p root filesystems... UML used to have some host kernel

• JoshTriplett 12y ago
  > Could you elaborate on 9p/virtio performance? I've had pretty good results booting qemu/kvm VMs with 9p root filesystems... The throughput is passable, if you do large enough reads; however, the latency for individual operations is terrible. Try booting via virtio-9p and building a kernel inside the virtual machine, as compared to the host system. Last time I tried it, it took several orders of magnitude longer, due in large part to the latency of simple operations like stat or open/read/close.
  • 4ad 12y ago
    On high-latency links, 9P is slow, but on low latency links (like should be provided by QEMU), 9P should be very fast. Looks like a QEMU implementation problem to me. That being said, calling QEMU's 9P "9P", is a stretch. 9P was designed wit multiplexing in mind, but it's not multiplexed in QEMU, in fact multiplexing in QEMU uses a completely different mechanism. You can't mount QEMU's thing in Plan 9 (unless someone added support for this when I wasn't looking).
    • JoshTriplett 12y ago
      I wouldn't find it surprising at all if the fault lies with qemu's implementation rather than with some inherent property of 9p. The performance test I ran was with qemu's virtio 9p implementation: compared to a kernel compile on the host, one in the guest took a few percent longer on a virtual block device, or hundreds of times longer on virtio-9p.