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Pros and cons of VTLs

Virtual tape libraries (VTLs), which treat disk as tape, offer two main advantages over disk-as-disk backup targets: ease of management and better performance. As described in Part 1 of this three-part series on the various ways to use disk to protect data (see D2D backup: Disk's dual role), a disk-as-disk target requires all of the usual provisioning steps of standard shared storage arrays. In contrast, if you tell a VTL how many virtual tape drives and virtual cartridges it should emulate, the VTL software automatically handles all of the provisioning and allocates the appropriate amount of disk to each virtual cartridge.

If the VTL needs to be expanded (not all VTLs are expandable), you simply connect the additional storage, tell the VTL it's there and the VTL will automatically begin using the new storage. There's no volume manager to run and no RAID groups to administer.

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Another important management advantage of VTLs is how easy it is to share VTLs among multiple servers and applications (see Should a virtual tape library be shared). To share a VTL among multiple backup servers running the same software, use the built-in library sharing capability that most commercial backup products have. To share a VTL among multiple servers running different apps, partition the VTL into multiple smaller VTLs, assign a certain number of virtual cartridges to each VTL and associate each VTL with a different backup server. Both of these scenarios are much easier than what's required to share a disk-as-disk target among multiple backup servers.

Better performance
To understand the performance advantages of VTLs, think of how backup apps write data to tape. A backup application typically continues writing to a tape until it hits the physical end of tape (PEOT). It will append to a tape, even if some of the previously written data has expired. Once the backup app hits PEOT, the tape is considered full. Most backup applications leave everything on the tape until all of the backups on that tape have expired; then they expire the whole tape and write to it from the beginning. Other backup applications wait until a certain percentage of the backups on a tape have expired before "reclaiming" that tape by migrating the non-expired backups to a second tape. The first tape is then expired and ready to be overwritten. The bottom line is that portions of a tape can't be overwritten.

This differs from how backup applications write to a file system. The application tells the operating system that it wants to write to a certain file name and then begins writing data to that file. Each backup gets its own file and when that file expires, it's deleted. The backup application has no knowledge of how this data is actually written to disk. Underneath the covers, the bytes of any given file are fragmented all over the disk, which results in performance degradation of the backup.

Because a VTL treats disk like tape, it eliminates fragmentation by writing backups to contiguous sections of disk. The blocks allocated to a tape stay allocated to that tape until the backup app starts overwriting that tape, at which point the VTL can once again write to contiguous sections of disk--just like data is written to tape. Because VTL vendors control the RAID volumes, they ensure that a given RAID group is only written to by a single virtual tape. A disk can perform much better if it's only writing/reading for a single application using contiguous sections of disk. This key difference explains why the fastest file systems write in hundreds of megabytes per second, while the fastest VTLs write in thousands of megabytes per second.

VTLs offer other advantages, as well. With one exception (see the next section), VTLs work with all existing backup software, processes and procedures (see NetBackup's inline tape copy, Do IBM Tivoli Storage Manager users need a VTL? and EMC/Legato's NetWorker understands disk, too). In other words, everything works exactly as it would with a physical tape library (PTL). That isn't the case with disk-as-disk targets, where backup software can behave quite differently.

VTL disadvantages
The disadvantage of VTLs most cited by storage admins is cost. They believe that if a disk array costs x, a disk array made to look like a VTL will cost x + y. But the y factor can vary from one VTL vendor to another. Most VTLs use capacity-based pricing, which means the cost is $x/GB. At least one VTL vendor uses throughput-based pricing, so the price is determined by the number of Fibre Channel (FC) connections. The actual price of VTLs with disk included ranges from less than $4/GB to a little more than $12/GB. Disk-as-disk units fall into roughly the same price range, so it's basically a misconception that a VTL will always cost more than a disk-as-disk device.

Another issue is the price of backup software licensing. If a VTL sits next to an existing tape library, it will most likely require an additional tape library license for a library that's actually not there. This adds to the price of the VTL. How much you pay is based on how the VTL is configured and how your backup software charges for libraries. Some backup software products have a single license for all tape libraries, while others charge for the number of slots or drives. When deciding how to configure your VTL, you should consider how your backup software charges for libraries. When comparing VTLs to disk-as-disk targets, you also need to remember that backup software products are beginning to charge to back up to disk-as-disk targets.

However, these licensing challenges will probably go away as backup software vendors move toward capacity-based pricing in an effort to appear more VTL friendly.

VTLs offering compression use in-band software compression that saves space, but results in a significant performance hit--as much as 50%. If your backup speed is throttled by the speed of your clients and/or network, you may not see this performance hit. But in local or LAN-free backups, speed tends to be most affected by the backup device. Some vendors perform their compression after the fact, attempting to give you the benefits of compression without the performance loss. As of this writing, only Quantum Corp. supports hardware compression that doesn't impact performance. It accomplishes this by using the same chip used in front of its tape drives.

This was first published in April 2005

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