As mentioned in comment #3, could this be related to a specific SATA chipset? I have a Intel 82801JI ICH10 SATA AHCI Controller, and have experienced random disk corruption on guests. Guests are both Linux and Win2k3. High usage on the sata controller seemed to be the trigger. Ive noticed no difference for if the guest is using virtio or ide mode. Guests will become unresponsive for >10 seconds when there is high disk usage on the node.