Yesterday I upgraded a Debian Lenny server to Squeeze. This server hosts two KVM guests. One has run Debian Squeeze itself all along, the other was originally Debian Lenny, upgraded to Squeeze the day before.

Everything appears to be working fine, however performance of the KVM guests is now quite poor. Everything seems to take much longer than it used to: logging in via SSH, checking for and installing updates with aptitude, requests to web applications running in a guest, etc.

Googling has left me the impression that much has changed in KVM from Lenny to Squeeze, and I also found some vague reference to there being a good way and a bad way to launch guests, but nothing concrete.

I am happy to provide configuration files, but rather than fill this question with noise, I'd like a little direction about where to look.

Pertinent packages are installed:

i   kvm                  - dummy transitional package from kvm to qemu-kvm
i   libvirt-bin          - the programs for the libvirt library
i A libvirt0             - library for interfacing with different virtualization
i A qemu-kvm             - Full virtualization on x86 hardware

kvm is Debian version 1:0.12.5+dfsg-5+squeeze6, qemu-kvm is 0.12.5+dfsg-5+squeeze6.

Here is the command for launching one of the guests, from ps:

/usr/bin/kvm -S -M pc -enable-kvm -m 768 -smp 1,sockets=1,cores=1,threads=1 -name apps -uuid 636b6620-0949-bc88-3197-37153b88772e -nodefaults -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/apps.monitor,server,nowait -mon chardev=monitor,mode=readline -rtc base=utc -boot c -drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -drive file=/raid/kvm-images/apps.qcow2,if=none,id=drive-virtio-disk0,boot=on,format=raw -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0 -device virtio-net-pci,vlan=0,id=net0,mac=54:52:00:27:5e:02,bus=pci.0,addr=0x3 -net tap,fd=35,vlan=0,name=hostnet0 -device virtio-net-pci,vlan=1,id=net1,mac=54:52:00:40:cc:7f,bus=pci.0,addr=0x4 -net tap,fd=36,vlan=1,name=hostnet1 -chardev pty,id=serial0 -device isa-serial,chardev=serial0 -usb -vnc -k en-us -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

  • Can you please post the versions of the installed kvm and qemu-kvm packages? – Valentin Aug 22 '11 at 18:51
  • Added: kvm 1:0.12.5+dfsg-5+squeeze6, qemu-kvm 0.12.5+dfsg-5+squeeze6. – Steve Madsen Aug 23 '11 at 1:15
  • Did you resolve those problems? – tholu Jul 31 '14 at 11:28

Thanks for posting the update about the packages and versions.

Well, it seems you are using the qcow2 image format. Various people have reported performance issues of kvm 0.12.5 in combination with qcow2, like here.

The reason why qcow2 behaves so badly with kvm 0.12.5 is that they changed the behavior of the image in case of crashes. qcow2 is now much more stable, but at the same time there is the side effect of a decrease in performance.

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