An enormous chunk of the money we spend on IT infrastructure goes to avoiding downtime -- from redundant power supplies and disks to clustering and redundant data centers. Yet we happily spend it. Lost productivity and shattered user confidence have a damaging effect on the ol' career prospects.
Of all of the advances over the past 10 years, server virtualization has probably done more than any other technology to decrease exposure to downtime. What server clustering once did for single database platforms, virtualization clusters can now do for any application you can run on them -- which will soon be just about everything.
Technologies like VMware's vMotion, which allows a virtual machine to be moved from one virtualization host to another without any perceptible downtime, and VMware High Availability (HA), which allows automated recovery from host failure, have gotten a lot of attention for their ability to eliminate or substantially decrease downtime windows. But if you ask me, one weapon in VMware's arsenal doesn't get the attention it deserves: Storage vMotion.
The Storage vMotion proposition
Storage vMotion (sVmotion, for short) allows you to move the underlying storage resources of a virtual machine from one volume or device to another without incurring downtime. Both the source and destination storage must be accessible from the same VMware vSphere host, so you can't use this feature to move a VM's storage and what host it's running on at the same time -- which means it isn't much use in environments that depend on direct-attached storage.