Virtual tape is a concept that has been in the data center for many years. Originally introduced for IBM mainframes, it is now exploding in the open systems arena. VTLs are logically just like physical tape libraries: They logically appear and operate as physical tape devices including virtual tape drives, data cartridges, tape slots, barcode labels and robotic arms.

A virtual tape library (VTL) is physically a highly intelligent optimized disk-based storage appliance. Because a VTL completely emulates a standard library, the introduction of virtual tape is seamless and transparent to existing tape backup/recovery applications.

Key Terms To Understanding VTL:

Virtual tape library
A VTL, or virtual tape library, is an archival backup solution that combines traditional tape backup methodology with low-cost disk technology to create an optimized backup and recovery solution.

Tape
A magnetically coated strip of plastic on which data can be encoded. Tapes for computers are similar to tapes used to store music.

Backup
To copy files to a second medium (a disk or tape) as a precaution in case the first medium fails. One of the cardinal rules in using computers is back up your files regularly.

Restores from tape are also time-consuming. Consider trying to recover a file that was part of a five-tape multiplexed backup. Each of the five tapes must be located in the library and loaded into tape drives. If the drives have tapes in them already, the tapes must be removed from the drives and moved to free slots before another tape may be loaded. Once the tapes are loaded, they must be advanced to where the file is and then, finally, the file can be read from tape. It can take many minutes just to start the recovery. If the tapes are not in the library, it can take many hours to recover a single file.

On the plus side, tape-based solutions are usually considered to be relatively inexpensive. But when the tape media is considered, the cost can skyrocket.

Some studies show that users will buy 30 times the amount of slots worth of tapes during the life of a library. For a medium-sized, 100-slot library, that's 3,000 tapes. LTO-3 tapes are currently in the $100 price range. Add the fact that extensive human intervention is required to manage and maintain a tape solution and they are not very inexpensive. It can cost a lot of money to be able to successfully backup your data 60 percent of the time.

Comparing the problems associated with tape solutions with a virtual tape solution shows how a VTL can change how a data center can be run.

VTL Performance
A virtual tape library solution can perform 10-times tape speeds for backups. Speeding up the backups will greatly shrink the backup window, which allows servers to be backed up faster. With existing backups finishing quicker, second- and third-tier servers that have not been backed up in the past may now fit into the backup schedule.

Recoveries are also significantly faster using a VTL (typically much faster than the backup). A single file can be recovered from a VTL faster than most tape libraries can find and load a tape into a drive. Full backups that span multiple tapes (as apposed to multiplexed) will also recover very slowly compared to a VTL since after the data is read from one tape the next tape must be located and loaded compared to a VTL that just keeps streaming data from disk.

All virtual tape loads are immediate so there is virtually no delay when a new "tape" is "loaded." People who resort to multiplexing backups to increase the performance of the backup are usually shocked to discover that their recovery times will be about twice as long as the backup was.

Most virtual tape library solutions also contain RAID-protected storage that has redundant, hot-swap components (drives, power, cooling). Backups that use a VTL rarely fail because of a VTL failure. Recoveries will never fail due to a bad or lost tape.

VTLs are just not prone to the types of failures that a traditional tape library has. For example, a backup to disk will never fail because of a bad tape, broken tape drive or broken robotics.

VTL Cost
The initial cost of a tape library can be less than the cost of a VTL. But when a three- or five-year cost-of-ownership is considered (tape media, failed backups, lost data due to failed recoveries, management costs, and so on) a VTL will be less expensive.

Also consider the lower cost of backup software. Some backup software is tiered based on the number of tape slots. By configuring a virtual library to have few slots, but very large "tapes," the software tier can be lowered. For backup software that is tiered on the number of tape drives, configure a virtual library with fewer "drives." Some backup software solutions are now adding a virtual tape library option that is priced based on the capacity of the library.

VTL Options
Today's virtual tape libraries range from a customer-supplied server with VTL software and separate disk to a completely productized solution where the server, software and disk are all bundled.