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Reducing VDI cost by exploring alternatives to centralized VM storage

By Amir Husain, president and CEO, VDIworks, special to Network World
April 09, 2012 03:51 PM ET

Network World - This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

One of the most significant cost elements with VDI is the centralized storage required for maintaining virtual machines. Hypervisor vendors argue that to get disaster recovery and high availability the additional expense is really a bargain. And in many instances that might be true. But in many cases cost is paramount, complexity is to be avoided and three or four nines of availability may be far down the list of priorities.

Availability and uptime are always important, but don't forget that moving from traditional desktops to a VDI architecture will improve uptime and availability regardless of whether centralized storage is used. So the real question is whether a fairly significant improvement in availability is good enough if it comes with low-cost, reduced complexity and fewer moving parts. Many IT shops will probably nod in the affirmative.

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Centralized storage is typically implemented with storage area networks that provide redundancy, network accessibility and management features that go beyond conventional, local storage. For example, SAN management tools allow you to add disks to your array(s) without discontinuing service. And hypervisors leverage these capabilities to deliver improved uptime and high availability through support of features like live virtual machine migration (also referred to as VMotion in VMware parlance).

In the event that a hardware failure occurs on the server running your VM, you can use these live migration techniques to transfer a VM to a different server, thus maintaining continuity of service. These features don't, however, make you immune to application crashes, OS crashes or many of the inherent instabilities in the OS and platform stack that don't have anything to do with hardware.

So, even with live migration and a SAN backend it's not as if you're getting complete immunity from downtime. Yes, hardware issues on your host server are eliminated from the downtime equation, but this improvement in uptime comes at a premium. SANs are costly when purchased from the category leaders. There are smaller companies offering more economic solutions, but regardless of the vendor, this class of storage is considerably more expensive when compared to distributed or local storage.

Distributed storage can be defined as self-contained islands of local storage associated with multiple servers, such that each local store remains accessible only to the server it is attached to, but where the aggregate capacity of all such islands can be used to store the total set of VMs required.

For example, if the servers are implementing a collection of hypervisors for VDI, then individual user VMs may find themselves mapped to specific local stores. The aggregate of all local stores is the distributed storage capacity. The big advantages are setup is simple, no SAN or specialized storage management tools are necessary, the server configuration is fairly simple to order and support and the cost per terabyte is much lower than for SAN based centralized storage solutions.

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