I recently had the opportunity to speak with Simon Crosby - CTO of XenSource, the Palo Alto, CA-based vendor that 'plays the dual role of leading the open source Xen [virtualization] community, while simultaneously selling value-added enterprise solutions based on Xen.'

Crosby cited Grid's problems handling relational data as a major challenge ...

"The Grid's I've seen in action don't deal with complex storage architectures -- the data sets are local to the computation or are made available from a centralized server. But for a broader class of enterprise applications, the storage problem must be addressed. Many applications are now interlinked ... for example, a Web service today is composed of a Web server, an app server, and a database."

... and had an interesting point about the difference between managing applications on the Grid, versus managing virtual machines on the Grid.

"We can instantly clone a running VM and distribute it across multiple servers in the datacenter. The Grid world understands how to use such features because the Grid community understands the concept of quickly creating multiple copies of an application on multiple systems. The key difference between scheduling an application to harvest resources from the Grid, and scheduling VMs onto computational and storage resources is that applications in the Grid world today tend to be of finite life-span (for example, consuming spare CPU to run a simulation) and will terminate at some point. But VMs are instances of operating systems, that essentially run forever until they are 'turned off.' VMs are more fundamental, perhaps more elemental than applications."