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Virtual Tape Library 101

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Virtual Tape Library (VTL) technology has been growing at a fast clip in recent years. According to IDC, the worldwide VTL market will reach $1.4 billion by 2011.

“Virtual tape technologies combine traditional backup methodologies with low cost disk technology to create a more efficient backup and recovery solution,” says Kathy Hodge, of Sun Microsystems Inc. of Santa Clara, CA.

VTL Basics

The big barrier in the way of disk-based backup has been high cost. Thus it has served a role as primary storage for years, with backup left to much cheaper tape systems. But as the price difference has narrowed, disk-based storage systems have blossomed.

Now several vendors provide VTL systems that use SATA disks that are much faster than tape. But it took a while for the technology to gain much traction.

“When virtual tape was first introduced to the IT industry, many storage managers dismissed it as a niche product not robust enough for enterprise class backups,” says Hodge. “Now this technology is proving itself in production environments.”

VTL essentially mimics a tape backup library but uses disks as the medium. It is an archival storage technology that makes it possible to save data as if it were being stored on tape although it is actually stored on disk or on another storage medium.

To the storage subsystem, VTL appears as tape libraries, but have the advantage of providing the performance of disk drives. To make things easy, data is written directly to disk drives in the same format as it would be written onto a tape cartridge. Thus there is no need to change the backup software when implementing VTL – everything is recognized as before.

By using disk, VTL eliminates the media and mechanical errors that are not uncommon with physical tape cartridges and tape drives. Since it can emulate more tape drives than a physical tape library has capacity for, more backup streams can run simultaneously. This enables service level goals to be more easily accomplished within the allotted window of time.

“A virtual tape library is a backup solution that uses an intelligent disk-based library that emulates traditional tape devices and tape formats,” says Hodge. “Data is written to disk just as it is written to a tape library, only faster.”

Virtual tape can be used as a secondary backup stage on the way to tape, or as a standalone tape backup. In disk-to-disk-to-tape (D2D2T), data is initially copied to backup storage on a disk storage system and then periodically copied again to a tape storage system. Virtual tape can also interface with the physical tape library to archive virtual tape volumes to physical cartridge tapes.

Read performance improvements, of course, are one big advantage of VTL. Physical tape data transfers are single threaded due to their sequential nature. VTL, on the other hand, can cope with multiple host data streams being writing to different disk buffer locations at the same time. In the event of an unplanned outage, for example, speed of recovery is essential.

Virtual tape allows for fast access to the data since it is stored on a disk buffer. That’s why virtual tape is being used so often as a backup staging location before the files are transferred to tape for long term data archival or offsite storage.

“There are occasions when the most recent backup has problems and the next older version is consequently required,” says Hodge. “This virtual tape storage approach allows for reduced effort and improved database availability, by avoiding costly physical tape restores for older versions of database backups.”

Fred Moore at Horison Information Strategies characterizes virtual tape as automated nearline storage category and recommends that 90 percent of data be stored there. Frequently utilized data and recent data would be stored online on the most expensive systems that offer the highest performance and the greatest levels of recoverability. Next comes nearline, which encompasses VTL and nearline tape. VTL offers the ability to retrieve the data within a few seconds and a reasonable price per GB. Various offline systems are also used for archiving.

“The cost per megabyte for virtual tape is about the same as a physical automated tape library, its accompanying drives and tape media, says Hodge. “However, for about the same price, shops are getting 10 times the performance plus the added benefits of software functionality down the road.”

This article was first published on

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