Virtual Tape Library (VTL) Explained
Virtual tape is a concept that has been in the data center for many years. It was originally introduced for IBM mainframes.
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.
Traditional Tape SolutionsTraditional tape devices have a few problems: They are slow and the only way to solve that problem is to add more and more tape drives. Tape library robotics are prone to failure and the tape media itself is "delicate" and must be stored in a conditioned, secure environment.
To increase backup performance, backups can be multiplexed across multiple drives and tapes. This increases the odds of a failed backup due to a bad tape, a faulty drive or malfunctioning robotics.
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.
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.
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.
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.
There are pros and cons for both extremes.
With an unbundled solution, the user gets to purchase each piece separately. The pieces include the VTL software, server, disk and potentially the SAN infrastructure. Unfortunately, the user must also purchase separate support agreements for the VTL software, server, disk and SAN infrastructure and each piece must be managed and monitored by the IT staff.
With a bundled solution, all the pieces are included, tightly integrated and guaranteed to work together. The solution is managed and monitored as one entity and support is covered by one contract.
With current bundled VTL solutions expanding to a petabyte or more, scaling the solutions is not an issue. Adding an additional VTLs for each petabyte of backup is acceptable for most environments. The only negative with a bundled solution is that it is bundled. Some people just do not like that. When it comes to backup, it is best to keep things simple so a bundled VTL solution is probably the best bet (there are not very many home-made tape libraries in production so why make your own VTL solution?).
Adding a virtual tape library into an existing tape environment will always improve the reliability of backups and recoveries. Even if a VTL is configured to backup only as fast as an existing library, the increase in performance for day-to-day data recovery makes the investment a no-brainer.
Add the robust data processing capabilities not available in physical tape libraries (e.g., replication, single-instance data storage) and a VTL can open the door for tremendous advances in tape backup methodology and revolutionize traditional operations.
Did You Know...
|Key Terms To Understanding VTL:
virtual tape library
Jim McKinstry is senior systems engineer with Engenio Information Technologies, an OEM of storage solutions for IBM, TeraData, Sun and others.
This article was originally published on June 16, 2006
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