Storage virtualization alleviates these problems by implementing a layer of abstraction between applications and physical storage, allowing storage to be combined and treated as a ubiquitous resource, regardless of location. While storage can be virtualized in several ways, the ultimate goal is to improve storage utilization (reducing or forestalling capital expenditures) and storage management. Aside from the obvious issues of pricing and support, here are the key points involved in purchasing storage virtualization products.
What does storage virtualization bring to the enterprise? Storage virtualization offers many potential benefits, but which ones does your organization need? Improved storage utilization, capital cost savings and ease of management are three common ones, but there are others that appeal to storage administrators. For example, virtualization can ease disaster recovery (DR) or business continuance planning by allowing the use of nonidentical hardware between sites. It can handle data migration between storage platforms, tiers and sites. Virtualization also eases storage capacity expansion, often automating the manual tasks needed to allocate storage to applications.
How much complexity will be added? Many virtualization products require additional hardware or software in the infrastructure. Beyond new servers or switches, determine what host device drivers, path managers, agents or shims you'll need to support the prospective virtualization product. Maintenance is usually first to feel the pinch here -- IT staff can easily get bogged down in patching and updating a bunch of storage virtualization servers when hardware is replaced or new versions become available. Not paying enough attention to maintenance can result in version disparity, leading to instability and performance problems. Evaluate any storage virtualization product from a management and maintenance perspective, and determine if the problems that it solves are outweighed by the new issues that it introduces.
Where does storage virtualization best fit in your environment? In terms of implementation, storage virtualization can be host-based, array-based or fabric-based. Host-based virtualization relies on software (installed on host servers) that monitors data traffic and storage. Veritas Storage Foundation from Symantec Corp. is a host-based virtualization product. Dedicated appliances, such as the File Director 7200 appliance from NeoPath Networks Inc., are similar. Array-based virtualization integrates the technology in the storage array itself, such as a TagmaStore array from Hitachi Data Systems. Growing attention is being paid to fabric-based virtualization, which runs dedicated software on intelligent switch devices. Each approach offers advantages and disadvantages in terms of performance, scalability and cost.
Scalability. A virtualization product can only manage a finite amount of storage. You should understand the tradeoff between scale and performance. This is particularly treacherous because many virtualization initiatives begin as test or pilot deployments before being deployed throughout the enterprise. Scaling issues may not appear until later in the deployment cycle. Evaluating scaling up-front can help weed out unacceptable products.
Interoperability. The promise of cross-vendor storage utilization has been a compelling feature of virtualization technology, but true heterogeneous support is still lacking in virtualization environments. Array-based virtualization typically locks in the array vendor. Host- and fabric-based virtualization products also impose a certain amount of vendor lock-in with the software or appliance that embeds the software. You should determine the compatibility of any virtualization products with your current environment, as well as likely upgrades or updates.
Test and start small. Most analysts recommend a thorough lab evaluation of any storage virtualization product prior to any purchase commitment. This should include a review of decommissioning drills. Once a purchase decision is made, you should start implementation on a small scale before building out the virtualization systematically. This conservative approach gives administrators ample time to get accustomed to virtualization management, and prevents unforeseen problems from crippling an entire data center.
Know how to undo the implementation. Storage virtualization isn't perfect. Performance issues, scalability limitations and interoperability problems are just a few reasons to decommission a virtualization product. An organization may also decide to discontinue one product in favor of a better one. Unfortunately, backing out of virtualization is disruptive to applications and confusing to administrators. Before committing to a virtualization product, discuss any back-out options with the vendor. ***