The virtual tape option for D2D backup/recovery

Virtual tape leverages hard disk technology while maintaining the manageability advantages of tape.

By Mike Parise

In the past, IT discussions about backup and recovery of critical data focused primarily on the cost of downtime. Downtime costs are still critical, but the focus now is on the value of data, and a different set of criteria has been established for assessing risk.

According to the Enterprise Storage Group (ESG), data value depends on its type and who is using it. For example, e-mail may lose 95% of its value after the first year, while database data may retain 75%. The new reality, which takes into account regulatory mandates and evolving business practices, is forcing organizations to re-evaluate their data-protection and recoverability practices.

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Backup-and-restore operations have become cumbersome; the performance advances of tape technology have not kept up with the business requirements of some companies; and tape libraries are susceptible to mechanical errors that can result in backup failures and restore delays.

There are a number of potential bottlenecks in current backup environments. The first place a bottleneck can occur is the LAN. To minimize bandwidth capacity issues, many organizations have built dedicated backup LANs to handle the backup traffic.

A second potential bottleneck is the backup application host—specifically, its processing power, bus speed, and port count. On the receiving end, you can add drives to a tape library and compress data to increase throughput. With compression, a tape drive might achieve 30MBps throughput, which translates to 108GB per hour, but that's still nine hours to back up 1TB. In addition to performance limitations, backup-and-recovery procedures can fail due to tape drive failures, misplaced cartridges, and accidental destruction of media.


The role of tape is evolving from a primary backup medium to an archiving medium. The Gartner consulting firm predicts that tape will be unsuitable as a restore media by 2008. The alternative to tape is disk-to-disk (D2D) backup/recovery, where disk drives are used to augment—or in some cases replace—tape. There are advantages and disadvantages to the various types of disk-based backup (see table).

Disk is not going to replace tape in the foreseeable future, and it should be considered as a complement to tape, providing improved reliability, performance, and data availability.

Technologies such as snapshots, third-mirror copy, disk-to-disk backup, and virtual tape improve throughput and recovery of data. In the snapshot scenario, data is moved off disk to tape daily to protect against device failure and data corruption, as well as to meet archiving requirements. Snapshots are different from third-mirror copy in that they save changes that point to the original data source; however, if the original disk fails, snapshots will not help you recover. In contrast, third-mirror copy creates an exact replica of data at a point in time and can be used to recover in case of failure.

Virtual tape

With virtual tape, data is pushed to what the backup application thinks is tape but is actually disk. Once it is on disk it can be managed as disk data; native replication applications can be used for disaster recovery, and additional data protection can be achieved through RAID. Also, virtual tape is transparent to the backup application and the overall backup-and-restore process. Using the existing backup application, operators can still define how long data is retained and when it is time to move it off disk to tape.

Archiving can be achieved by sending a cloning command from the backup application to the virtual library. Data is then moved from virtual cartridges to an actual tape library. The operation is controlled by the backup application. Once data is copied from disk to tape, tape cartridges can be moved off-site. Data remaining on disk offers improved recoverability for a designated period of time.

In addition to archiving (which can be used as a component of a disaster-recovery solution), since the backup data is on disk, native replication software can be used to move data from one location to another. Once data is at a different site, it can be moved to tape and recovered for uses such as disaster recovery, staging sites, and development testing.

Backup to disk addresses performance problems, but it does not address some of the other issues IT administrators face, such as managing data on the backup media, tracking backups from each application or department, and keeping existing processes in place. With virtual tape, organizations can take advantage of disk technology while maintaining the manageability of tape.

Virtual tape offers improvements in throughput, manageability, and scalability and enables automation of operations and elimination of operator variables such as cartridge handling, loading, and storing. IT managers can leverage virtual tape technology to improve backup-and-recovery operations, enhance performance, meet recovery point objectives, and eliminate some of the risk associated with the mechanical nature of tape.

Mike Parise is vice president of sales and marketing at Diligent Technologies (www.diligent.com) in Framingham, MA.

This article was originally published on October 01, 2003