The benefits of virtualization don't end with consolidation. Storage virtualization offers the advantage of aggregation, too, which allows disparate or isolated storage resources to be pooled, provisioned and allocated regardless of their physical location within the enterprise. Storage can be virtualized in a variety of ways to achieve a balance of performance, management efficiency and flexibility.
Virtualization directly affects storage efficiency. As storage needs increase and storage systems proliferate across an enterprise, unused storage resources can be "orphaned" through overprovisioning or neglect. This waste often leads organizations to invest in additional storage sooner than necessary, resulting in higher costs.
Overprovisioning is the act of assigning more storage space to an application than necessary. The practice is meant to prevent the provisioned volume from running out of space, which would force administrators to provision new space and migrate the exhausted volume to a larger space. If the overprovisioned space goes unused, this practice can be extremely wasteful and is therefore discouraged.
Virtualizing storage combines disparate sources, which reduces waste, improves utilization rates and forestalls the costs of additional disk investments. For example, nonvirtualized storage use often peaks around 50%, while virtual storage utilization can frequently exceed 80%.
Virtualization can also improve storage management. Without virtualization, each heterogeneous storage system must be managed through a console that's unique to that particular storage system or manufacturer. Because management differences can exist between products from the same vendor, each new storage platform could introduce more management overhead.
"When you move to storage virtualization, that becomes the central point of storage administration and management," said Ray Lucchesi, president and founder of Silverton Consulting Inc., an independent technology consulting firm headquartered in Broomfield, Colo. "Performance reports, configuring LUNs … it's all done through the storage virtualization console."
Storage flexibility, also called storage agility, is another important benefit. Storage virtualization allows data to be migrated or copied nondisruptively across storage systems. Data can also move offsite across a WAN. For example, data files and complete virtual machines (VMs) can be moved from one storage system to another within the data center. This may make sense when one storage system needs to be maintained or upgraded. Similarly, virtualized storage volumes can be moved to -- or retrieved from -- offsite storage at a disaster recovery (DR) site.
"Once we start to take data from multiple systems, or even data on one server, we get it onto our SAN and we open up quite a bit of opportunities to push that data or stream it to an offsite storage facility as well," said Scott Gorcester, president of Moose Logic, an IT services firm headquartered in Bothell, Wash.
Downsides of storage virtualization
One issue with storage virtualization is that the abstraction layer introduces complexity. Just as the hypervisor in server virtualization obscures the relationship between physical servers and the VMs that run on them, storage virtualization obfuscates the relationship between pooled storage and underlying disks or storage subsystems. Virtual LUNs do not necessarily correlate to physical storage. This can complicate troubleshooting and problem resolution, especially as LUNs are migrated and copied between storage systems.
This type of abstraction can also prove problematic when storage virtualization is used temporarily. While most organizations approach storage virtualization as a permanent technology, it's also possible to use virtualization to assist with storage migration between physical systems.
"Depending on how the storage behind a virtualization engine is used, there may be some [manual] migration activity required to isolate the storage," said Lucchesi. Organizations that choose to remove a virtual storage infrastructure may encounter problems re-establishing relationships between applications and stored data.
Experts like Gorcester do, however, question reliability. Using multiple diverse storage systems carries some amount of natural redundancy -- a failure in one storage system may not affect availability of other applications, data or VMs. By aggregating storage through virtualization, a problem with one storage system can affect several other virtual LUNs.
"If it [a storage system] were not designed properly, we could get to a scenario where there's just not enough resiliency to the infrastructure," Gorcester said. "We really need to build in a high level of availability, resiliency and geographic distribution."
Finally, organizations must be sensitive to vendor lock-in. Interoperability between storage virtualization products is often acceptable, but not necessarily universal. Organizations that are evaluating or selecting a storage virtualization platform should consider its interoperability across all of their storage systems, as well as its compatibility with other virtualization products in place.
"If a lock-in impedes your business, costs you financially, costs you functionally, costs you extra complexity, then lock-in could be bad," said Greg Schulz, founder and senior analyst at Stillwater, Minn.-based StorageIO Group, an IT infrastructure technology analyst and consulting firm. "If that vendor lock-in enables you to be more agile or introduces new technology in a cost-effective manner, then lock-in is not a bad thing."
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