Storage Virtualization Options: The Pros And Cons

In the second of a two-part series on storage virtualization and based on a new InformationWeek report, Strategy: Storage Virtualization Guide, Network Computing takes a closer look at the pros and cons of the available storage virtualization options. Part 1, The Importance Of Storage Virtualization," examined the various places within the IT infrastructure where storage virtualization can take place, including servers and disk arrays, and via physical and virtual appliances.

One could assume that the dynamic duo of EMC and its majority-owned VMware, the leaders in storage and virtualization, respectively, would have an unfair advantage in storage virtualization. Not so, says Howard Marks, founder and chief scientist at DeepStorage.net, a storage and networking consultancy, and a frequent contributor to Network Computing and InformationWeek.

"Storage virtualization and server virtualization are separate, and EMC is more focused on building features into the storage platforms than in VPLEX," he states. "That said, VMware and the other hypervisor vendors are adding storage virtualization features to the hypervisors. Storage vMotion is, after all, automated migration, and VMware SRM's replication is hardware-agnostic."

Solid-state disks (SSDs) have started showing up in storage virtualization vendors' lineups to accelerate storage access. A number of vendors, including FalconStor NSS and NexentaStor, can use flash in the virtualization appliance as a cache, while others, like IBM's SAN Volume Controller (SVC) and DataCore's SANsymphony, will combine SSDs and disks to create storage pools with automated sub-LUN tiering between the SSD and disk.

Storage federation, which is EMC's VPLEX claim to fame and available in IBM's SVC, extends the data protection aspect of data replication to turn a set of independent disk arrays into a single federated storage system across multiple data centers. Federation goes two steps further than the data protection provided by conventional replication, Marks says: It allows access to the volume at both the primary and secondary systems, and maintains the same volume identity across multiple systems. Virtual machines and the applications that run on top of them can be moved from site to site in a federation system.

Evolving from the popular Veritas Volume Manager, Symantec's Storage Foundation has added snapshots, thin provisioning, multipath management, SSD acceleration and just about any other storage management feature you can think of, notes Marks. VMware vSphere doesn't include a volume manager, per se, but many of its storage features, such as thin provisioning, solve the same problems as more traditional storage virtualization options.

Dedicated storage appliances such as EMC's VPLEX, IBM's SVC and FalconStor's NSS combine storage virtualization software with industry-standard x86 servers. Marks says others, like NetApp's V-Series, use custom hardware, while DataCore's SANsymphony-V is a software product that runs on Windows Server.

A more recent development is the introduction of the virtual storage appliance (VSA), software that performs some of the same functions as a physical appliance but is instead delivered as a virtual machine that runs on a hypervisor. Marks says there's a wide range of functionality in the VSAs available today--StarWind, Openfiler and NexentaStor are virtual server versions of the vendor's software that was originally designed to turn a physical server into iSCSI targets or unified storage systems. Others, like StorMagic and VMware VSA, were designed specifically for the hypervisor environment, while some, such as EMC and NetApp, have limited-use or limited-distribution VSAs that emulate their hardware storage systems.

The bottom line, says Marks, is whether it's implemented as a dedicated appliance or as a feature of a storage array or hypervisor, storage virtualization can make both you and your storage system more efficient. It can make your life easier through transparent migration and a consistent user interface across multiple storage systems. Or it can reduce the amount of extra storage you keep around just in case, by boosting your storage utilization through thin provisioning and snapshots.

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