New KVM deployment bugs and recommendations (Ubuntu 14.04: qemu 2.0, libvirt 1.2.4, Linux 3.10)

New Linux KVM qemu deployment, running on Ubuntu 14.04 with Linux 3.10 kernel and openvswitch. Hardware setup is 2 SSD in RAID1, and 2 7200RPM HDD in RAID1 using mdadm. bcache is being used as the backing cache for the HDD.


  • hv_vapic ("vapic state='on'" in libvirt) causes Windows 2008 R2 and above VMs not to boot if CPU is an Intel IvyBridge or greater (check /sys/module/kvm_intel/parameters/enable_apicv) – Redhat Bugzilla
  • Linux 3.12 or greater (Ubuntu 14.04 ships with 3.13) have issues with virtio-net NIC and TSO (RX and TX checksuming) offloading – TCP sessions can't be established across virtual machines in certain situations (think a virtual machine as a firewall) – Debian Bugreport
  • Windows virtual machines still freeze up/high latency if you use virtio NIC, this is with the latest signed drivers available from the Fedora Project
  • Still have issues with "Russian roulette" of network interfaces with openvswitch – Blog post


Installed Packages

apt-get install haveged ntp sysstat irqbalance acpid
Linux KVM, openvswitch, virt-install, virt-top
apt-get install qemu-kvm libvirt-bin virtinst virt-top openvswitch-switch sysfsutils iotop gdisk iftop
apt-get install python-software-properties
add-apt-repository ppa:g2p/storage && apt-get update && apt-get install bcache-tools

Tuning memory, scheduler I/O subsystems for Linux KVM

Taken from RHEL 6 tuned (virtual-host)


Disable experimental virtio-net zero copy transmit

RHEL 7 has experimental_zcopytx disabled by default.

options vhost_net  experimental_zcopytx=0

Use virtio-blk for guests, and enable Multiqueue virtio-net (except Windows)

Linux KVM page describing Multiqueue

  <interface type='network'>
    <model type='virtio'/>
    <driver name='vhost' queues='4'/>

Where number of queues is equal to the number of virtual processors assigned to the virtual machine. Don't forget to enable the vhost_net kernel module, edit /etc/default/qemu-kvm and set VHOST_NET_ENABLED=1.

Make sure to enable Multiqueue support in the guest

ethtool -L eth0 combined 4

Use deadline scheduler, and enable transparent hugepages for KVM

GRUB_CMDLINE_LINUX_DEFAULT="elevator=deadline transparent_hugepage=always"

Don't forget to run update-grub to make the changes persistent.

For Windows guests, take advantage of Hyper-V enlightments and use e1000 Ethernet adapter

Linux KVM presentation on Hyper-V enlightenment (slightly outdated)

  • hv_vapic (for "supported processors") for Virtual APIC
  • hv_time (aka "hypervclock") for TSC invariant timestamps passed to guest
  • hv_relaxed to prevent BSOD under high load (when a timer can't be serviced when expected)
  • hv_spinlocks let's the guest know when a virtual processor is trying to acquire a lock on the same resource as another processor
    <relaxed state='on'/>
    <vapic state='on'/>
    <spinlocks state='on' retries='4096'/>
<clock offset='localtime'>
  <timer name='hypervclock' present='yes'/>
  <timer name='hpet' present='no'/>

Build and install longterm Linux 3.10 kernel for stability (and working openvswitch with virtio-net)

apt-get -y install build-essential
cd /usr/local/src
tar -Jxf linux-3.10.44.tar.xz
cd linux-3.10.44
cp /boot/config-`uname -r` .config
make olddefconfig
make -j`nproc` INSTALL_MOD_STRIP=1 deb-pkg
dpkg -i ../*.deb
apt-mark hold linux-libc-dev

Time keeping is king on FreeBSD – TSC and "how not to have time go backwards in guest"

<clock offset='localtime'>
  <timer name='rtc' tickpolicy='catchup'/>
  <timer name='pit' tickpolicy='delay'/>
  <timer name='hpet' present='no'/>

openvswitch and libvirt: vnet port "russian roulette" on restart (solution)

Update: This issue has been resolved in libvirt 1.2.7 release, or commit. The below instructions are no longer required if your distribution has updated the package.

libvirt has openvswitch integration. When a virtual machine is started that is using openvswitch for the network port, a vnetX interface is created (where X is an incremental number, from 0) on start and destroyed on shutdown by libvirt. openvswitch's configuration is persistent, being that the vnetX interface created by libvirt is saved to a database and will be available on the following reboot.

As outlined in my bug report submitted in September 2013, this quickly breaks down if libvirtd is shutdown after openvswitch because libvirt can't delete the port it's created or the machine is restarted/shutdown incorrectly. If you have virtual machines that are on different VLANs, or interfaces you can quickly have them being assigned to the wrong virtual machine as libvirt doesn't error out if the interface already exists when it tries to create it (imagine swapping around LAN and WAN ports on a firewall.)

I solved this by adding creating an upstart job override on the Ubuntu LTS releases in /etc/init/openvswitch-switch.override:

post-start script
    ovs-vsctl show | grep 'Port \"vnet[0-9]*\"' | awk -F\" {'print $2'} | xargs -I {} ovs-vsctl del-port {} || :
end script

I've tested this issue and proven it's existence in OpenSuSE 12.3 (Dartmouth), Debian (stable) and Ubuntu 12.04/14.04 (LTS) distributions.

I/O caching under QEMU KVM virtualization on Linux

Caching modes in QEMU

Mode Host page cache Guest disk write cache
none off on
writethrough on off
writeback on on
unsafe on ignored


  • device.virtio-disk0.config-wce=off (qemu) or config-wce=off (libvirt) prevents guest from setting the write cache
  • Use cache=none for local RAW storage, cache=writethrough for NFS/iSCSI backed storage

Networking with a gateway not on the local subnet on NetBSD at OVH

NetBSD has a FAQ for networking that outlines how to do Networking with a gateway not on the local subnet, unfortunately the recipe that they provide doesn't actually work "in the real world." The route command they provide does not make the network stack send an ARP who-has for the IP address and requires that you statically set the MAC address of the gateway.

I figured out a work-around for this, based on some insight from people on the NetBSD tech-talk mailing list. This allows you to use NetBSD as a guest operating system on providers such as OVH and Hetzner:

# ifconfig fxp0 inet 
# route add -net -cloning -link fxp0 -iface 
# route add default -ifa

The trick was to specify use route cloning, and use a net definition instead of a host definition. Now NetBSD will send an ARP who-has request for the gateway IP address.

To supplement the OVH bridge client guide that is available on their Wiki, it would fit into the following template:

# ifconfig fxp0 inet Fail.over.IP netmask broadcast Fail.over.IP 
# route add -net Your.Server.IP.254/32 -cloning -link fxp0 -iface 
# route add default -ifa Fail.over.IP Your.Server.IP.254

This should allow you to use NetBSD as a guest and not get blocked by OVH robots that check for too many ARP requests.

What Linux/*BSD distributions have Syncookies enabled by default?

In light of the recently published article on Quick Blind TCP Connection Spoofing with SYN Cookies, I wanted to see what operating systems and distributions have Syncookies enabled by default.

Distribution Sysctl Default
Ubuntu Linux 12.04 net.ipv4.tcp_syncookies On
Debian Linux 6 Off
Debian Linux 7 On
CentOS 5 On
CentOS 6 On
FreeBSD 8 net.ipv4.tcp_syncookies On
Solaris 10 Not Implemented Off
OpenBSD 5.3 Not Implemented Off

I'm not sure that turning off Syncookies is the best idea, due to the potential DoS effects from disabling them – applications should use something besides IP addresses for authentication.