[Users] VirtIO disk latency

[Markus Stockhausen]

Hello,

coming from the "low cost NFS storage thread" I will open a new one
about a topic that might be interesting for others too.

We see a quite a heavy latency penalty using KVM VirtIO disks in comparison
to ESX. Doing one I/O onto disk inside a VM usually adds 370us of overhead in
the virtualisation layer. This has been tested with VirtIO-SCSI and windows
guest (2K3). More here (still now answer yet):

http://lists.nongnu.org/archive/html/qemu-discuss/2013-12/msg00028.html

A comparison for small sequential 1K I/Os on a NFS datastore in our setup gives:

- access NFS inside the hypervisor - 12.000 I/Os per second - or 83us latency 
- access DISK inside ESX VM that resides on NFS - 8000 I/Os per second - or 125us latency
- access DISK inside OVirt VM that resides on NFS - 2200 I/Os per second - or 450us latency

Even the official document at http://www.linux-kvm.org/page/Virtio/Block/Latency
suggest that the several mechanisms (iothread/vcpu) at least have a overhead
of more than 200us

Has anyone experienced something simlar. If these latency are normal it would 
make no sense to think about SSDs inside a central storage (be it ISCSI or NFS 
or whatever).

Markus

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