Virtual tape evolves to survive

This tip explains how virtual tape libraries (VTLs) work and what they are useful for. It also explores changes currently occuring in the VTL world.

What you will learn from this tip: Information about virtual tape libraries (VTLs) and the changes currently occurring in the VTL world.


Disk backup has arrived, and it's not going anywhere anytime soon. But VTLs have a long and winding road ahead of them.

In a nutshell, a VTL works by "faking out" the backup software and presenting a disk array as a tape device, says Frank Slootman, president and CEO at Data Domain, Palo Alto, Calif., which makes a disk-based backup system. This is useful if your backup software doesn't natively support disk targets, or if you're making inefficient use of your tape devices and need to improve your backup performance.

The list of VTL-based disk backup products is long, and includes ADIC's Pathlight VX, EMC's Clariion Disk Library, Quantum's DX-Series, StorageTek's Virtual Storage Manager (VSM) and VSM Open, and the latest entrant, Hewlett-Packard's StorageWorks 6000 Virtual Library System. There's also a bevy of smaller firms peddling VTL products, including Diligent Technologies, FalconStor (which also resells its software through third parties), Neartek and Sepaton. Backup software vendors such as Arkeia and Atempo include VTL features in their software, and others are rumored to be following suit.

Related information

Tech Report: Virtual Tape Libraries

Learning Guide: Backup/recovery

Tech Roundup: D2D backup

But for some customers, VTL just doesn't cut it. Take Scott Roemmele, for example, SAN engineer team leader at Quicken Loans, an online mortgage lender in Livonia, Mich. Last year, his company began looking for a way to speed up mailbox restores for its bankers -- a quest that led Quicken Loans to look at different disk-based backup products. "We've found that most mailbox restores happen within 14 days of deletion, so if I can keep up to 21 days of backups online, I don't have to go to tape," says Roemmele.

The VTL products he looked at, however, were priced too high to justify multiweek retention times, and the enhanced tape-media utilization VTLs provide didn't bring enough benefit either. "A VTL was just too costly for what it brought to the table," says Roemmele. "It seemed kind of frivolous." Instead, Roemmele installed a DD200 from Data Domain which, thanks to specialized capacity optimization technology, can store approximately 10 terabytes (TB) of data -- or 14 to 18 days of Exchange backups -- on only 1 TB of physical hard disk drives.

However, VTLs aren't only about enabling longer retention periods -- they're also about achieving better backup performance than you can get with generic disk arrays. "You can't just put a disk on the network and call it a backup," says Shane Jackson, director, enterprise product marketing and strategic alliances for Quantum's storage systems business unit. "That can cause more problems than it solves." Backup data, he explains, is written sequentially; if written to a random-access device like disk, it can result in poor performance and disk fragmentation. A good VTL package compensates for those discrepancies and can deliver optimal performance with the fewest possible tape drives. Quantum's DX30 and DX100, for example, are designed to back up 1 TB and 2 TB of data per hour, respectively, reports Jackson.

Furthermore, their tight integration with standard-issue backup software such as Veritas NetBackup, make VTLs a great "risk mitigation play," says Ray Anderson, director of IT at Egenera Inc., a blade server vendor in Marlboro, MA. Egenera is a Sepaton customer, and internal tests have concluded that the presence of the VTL appliance will dramatically improve restore times, although Egenera has yet to try and do a real restore.

VTL vendors have also started to heed the call for greater capacities. Sepaton, for example, has increased capacity with its latest VTL appliance, the S2100es, which goes beyond the previous limit of 200 TB all the way to 1 petabyte (PB). Furthermore, the appliance supports software-based compression, which compresses the data by roughly 2:1.

Quantum has added optional hardware-based compression to its DX30 and DX100 VTLs, which according to Jackson, enables customers to increase their backup retention periods to 30 days or more.

These are wise moves, says Tony Asaro, senior analyst at the Enterprise Strategy Group, Milford, Mass., but in the long term, they probably won't be enough. "The VTL guys will have to add more and more capabilities if they're going to make it more than just [tape] emulation," he says. Besides capacity optimization, Asaro sees a need for additional replication capabilities and features such as synthetic full backups. "Unless the VTL guys start doing this...it's going to be a stop-gap technology."

However, it will be a very long time before VTL becomes extinct, if ever, says Brad O'Neill, senior analyst and consultant at Taneja Group, Hopkinton, Mass. "The complete replacement of tape-centric technologies will take the better part of a decade," he says, during which time "we'll have a co-existence of VTL disk technologies alongside pure, disk-as-disk-based data protection solutions."

For more information:

Pros and cons of VTLs


This was first published in July 2005

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