[PVE-User] Slow speeds when KVM guest is on NFS

[Mikhail]

Hello,

Please help me to find why I'm seeing slow speeds when KVM guest is on
NFS storage. I have pretty standard setup, running Proxmox 4.1-1. The
storage server is on NFS connected directly (no switches/hubs, direct
NIC-to-NIC connection) via Gigabit ethernet.

I just launched Debian-8.3 stock ISO installation on the KVM guest
that's disk resides on NFS and I'm seeing some terribly slow file copy
operation speeds on debian install procedure - about 200-600
kilobyte/second according to "bwm-ng" output on storage server. I also
tried direct write from my Proxmox host via NFS using "dd" and results
are showing near 1gbit speeds:

# dd if=/dev/zero of=10G bs=1M count=10000
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB) copied, 115.951 s, 90.4 MB/s

What could be an issue?

On Proxmox host:

# cat /proc/mounts |grep vmnf
192.168.4.1:/mnt/vmnfs /mnt/pve/vmnfs nfs
rw,relatime,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=192.168.4.1,mountvers=3,mountport=47825,mountproto=udp,local_lock=none,addr=192.168.4.1
0 0

storage.cfg:
nfs: vmnfs
	export /mnt/vmnfs
	server 192.168.4.1
	path /mnt/pve/vmnfs
	content images
	options vers=3
	maxfiles 1

KVM guest config:
# cat /etc/pve/qemu-server/85103.conf
bootdisk: virtio0
cores: 1
ide2: ISOimages:iso/debian-8.3.0-amd64-CD-1.iso,media=cdrom
memory: 2048
name: WEB
net0: virtio=3A:39:66:30:63:32,bridge=vmbr0,tag=85
numa: 0
onboot: 1
ostype: l26
smbios1: uuid=97ea543f-ca64-43ab-9d66-9d1c9cd179b0
sockets: 1
virtio0: vmnfs:85103/vm-85103-disk-1.qcow2,size=50G

Any suggestions where to start looking is greatly appreciated.

Thanks.