11:16 a.m.
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Hello,=0A=
=0A=
coming from the "low cost NFS storage thread" I will open a new one=0A=
about a topic that might be interesting for others too.=0A=
=0A=
We see a quite a heavy latency penalty using KVM VirtIO disks in comparison=
=0A=
to ESX. Doing one I/O onto disk inside a VM usually adds 370us of overhead =
in=0A=
the virtualisation layer. This has been tested with VirtIO-SCSI and windows=
=0A=
guest (2K3). More here (still now answer yet):=0A=
=0A=
http://lists.nongnu.org/archive/html/qemu-discuss/2013-12/msg00028.html=0A=
=0A=
A comparison for small sequential 1K I/Os on a NFS datastore in our setup g=
ives:=0A=
=0A=
- access NFS inside the hypervisor - 12.000 I/Os per second - or 83us laten=
cy =0A=
- access DISK inside ESX VM that resides on NFS - 8000 I/Os per second - or=
125us latency=0A=
- access DISK inside OVirt VM that resides on NFS - 2200 I/Os per second - =
or 450us latency=0A=
=0A=
Even the official document at http://www.linux-kvm.org/page/Virtio/Block/La=
tency=0A=
suggest that the several mechanisms (iothread/vcpu) at least have a overhea=
d=0A=
of more than 200us=0A=
=0A=
Has anyone experienced something simlar. If these latency are normal it wou=
ld =0A=
make no sense to think about SSDs inside a central storage (be it ISCSI or =
NFS =0A=
or whatever).=0A=
=0A=
Markus=0A=
=0A=
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