Storage VMotion

I read the following interesting news release from VMware today. It covers lots of new features for the future release (3.5) of VMware but the one that caught my eye discusses a new feature called VMware Storage VMotion.

This feature will apparently allow the migration of a VMware virtual disk between different storage systems in the same manner that VMotion allows a virtual host to be moved between physical servers. I'm interested in how VMware will have chosen to implement this as there are lots of places in the stack they could choose to do it. For example, will there be any integration with array vendors' technology (like SRDF/TrueCopy) to manage the replication at the lowest level? Will the replication be managed via a virtual target/initiator in the fabric or will the VMware O/S manage the duplexed data writes at the application layer?

The difficulty of using replication outside of the application layer will be managing data integrity and also speed of replication cutover as the VMware guest is shifted to the new physical location. Add into that the complexity that each member of the VMware "cluster" will probably want read/write access to the virtual disks on which the virtual hosts are defined, then technologies like SRDF aren't going to work.

What about the CDP products? These seem to be a good logical fit as they replicate and track each block change independently, but I think the same issues of read/write access will apply and therefore these products will be equally unsuitable.

I think it is likely VMware will implement a "standard" cluster with multiple disks being written to by all members of the cluster and using IP to manage synchronisation. This may be good for a local solution but in reality what does VMotion then buy you? As a tool for managing the location of virtual machines across a farm of servers then VMotion is a fantastic tool. I just love the ability to move a host around to manage performance/workload on physical machines and to provide the ability to take physical servers out of a complex in order to do maintenance work.

But with today's 99.999% available storage subsystems, which can be maintained and expanded online without outage, is there any benefit in being able to move a VMware host to another storage system, unless that storage system is remotely located?

Storage VMotion sounds like a great idea but I'm not sure of the practical use of it - especially bearing in mind there will be a significant cost associated with the new feature.