From Alessandro Perilli: http://cloudcomputing.info/en/news/2010/07/yahoo-to-move-to-xen-for-cloud-computing.html
In a roundtable hosted by Association for Computing Machinery (ACM) that took place last month, the Vice President of Cloud Computing at Yahoo! Surendra Reddy revealed that the search giant is adopting the open source hypervisor Xen.
The information was provided during a discussion about the challenges of networking management inside cloud computing infrastructures:
We are moving to Xen and building a new data-center architecture with flat networks. We tried to use VLANs, but we have taken a different approach and are going to a flat layer 2 network. On top of this we are building an open vSwitch model placing everything in the fabric on the server.
My problem is in responding to the service requirements of my applications and addressing things such as latency and throughput. The data needed to address these issues is not available from either a network virtualization solution or the hypervisor.
Also, my uplink from the switches is 10 gigabits per second or multiple 10 gigabits per second, but my NICs are only one gig. If I run 10 VMs on a box, then all of the bandwidth aggregates on one or two NICs.