Enterprise requirements
Definitions of an enterprise VTL environment vary, but vendors and analysts generally agree that an enterprise-class VTL should back up at least tens of TBs of data daily and be able to store hundreds of TBs of data. These high-end VTLs also need to scale easily and support complex backup and recovery environments, such as those that include multiple remote offices or multiple VTLs in the storage fabric.

The ability to easily add disk capacity is important not only because organizations are backing up more and more data all the time, says Lauren Whitehouse, an analyst at Milford, MA-based Enterprise Strategy Group (ESG), but because companies often buy their first VTL to meet the needs of a specific department and then add to it as they see the value it provides.

Because of the number of production servers involved, failed backups are considered unacceptable in an enterprise VTL environment, says Andrei Shishov, VP of backup platforms engineering at EMC Corp. Reliability is often provided through features such as clustered failover among VTL nodes, as well as redundant components such as power supplies within individual nodes.

Performance is another basic requirement for an enterprise VTL, so it can back up data quickly enough to fit within an organization's backup window and restore files quickly when needed, says Whitehouse. Exact definitions of what is sufficient performance vary, although Peter Eicher, director of product marketing at FalconStor Software Inc., estimates the minimum at 300MB/sec to 400MB/sec. In addition to improving reliability, clustering can boost performance by spreading the work of reading and writing data, and/or deduplicating it, among multiple VTL servers.

Because enterprise customers "potentially have not only the local data center, but maybe some remote offices to be concerned about," support for a variety of backup applications is also a must-have, says Whi...

The desire to cut the purchase and energy costs of disk drives, and to reduce the bandwidth required to replicate data among various sites (such as for DR), have made deduplication a must-have feature for enterprise VTLs (see "Dedupe options," below). Some vendors are also introducing massive array of idle disks (MAID) or spin-down features that power up disks only when they read or write data.

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Deduplication can be done inline (as the data is taken into the virtual tape library [VTL]) or post-processing, after it's written to disk. Inline reduces the amount of disk space required but can slow performance; post-processing requires the most disk space, but can speed backups by allowing the disks to work at full speed as data is written to them.

Many vendors offer their own spin on deduplication, seeking to reduce the amount of time, processing power or disk space consumed by the dedupe process. Diligent Technologies Corp. (an IBM Corp. company) first identifies similarities among the data being backed up, and then submits only those similarities for detailed byte-by-byte deduplication. IBM Corp. will incorporate Diligent's software into its own products, says Tom Grave, Diligent's director of product management. EMC Corp. recently added dedupe functions (licensed from Quantum Corp.) to its VTLs.

Quantum's DXi7500 high-end VTL asks a user to choose parameters such as their backup window and the amount of data they need to back up, and then automatically chooses whether to perform deduplication inline or after the data is backed up, says Mike Sparkes, Quantum's product marketing manager for enterprise disk systems.

While many deduplication vendors boast of deduplication ratios of 20:1, says Sparkes, Quantum's customers report data-reduction ratios of anywhere from 5:1 or 6:1 to 30:1 or 40:1. Just how much space deduplication will save a given customer varies based on the amount of data being stored, how long it's kept and how often full backups are done, as well as the deduplication technology being used.
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