Feature

Pros and cons of VTLs

Should a virtual tape library be shared?
Partitioning makes it possible to share a virtual tape library (VTL) among backup servers running the same application; however, this can increase costs if your backup software charges by the drive. For example, assume you have seven servers, each of which needs 10 tape drives once a week for their full backup. You could create 10 virtual tape drives and share them, or you could create 70 virtual drives and give each server the 10 tape drives it needs. Unlike a physical tape library, a VTL can do this with no problem. But if your backup software charges by the drive, that's a 700% increase in tape drive costs.

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NetBackup's inline tape copy
Veritas NetBackup supports a feature called inline tape copy, which allows sending a backup to two tape drives simultaneously--creating an original and copy in one step. An alternative is to use a standalone VTL, and to send one copy to physical tape and one to the virtual tape library. The shortcoming with this approach is that it causes the VTL to run at the speed of the tape drive--defeating the purpose of going to disk backup in the first place. A more interesting approach would be to use an integrated VTL, send both backups to virtual tape, and then use the integrated VTL to copy one to physical tape.
Do IBM Tivoli Storage Manager users need a VTL?
While IBM's Tivoli Storage Manager (TSM) backs up directly to disk quite well, TSM administrators will experience provisioning and fragmentation issues if they begin storing all onsite backups on disk. (Most TSM disk storage pools aren't fragmented because they're immediately migrated to tape every night.) So, the advantages of virtual tape libraries (VTLs) apply to TSM as much as they apply to other backup products. In addition, a VTL would let TSM users create thousands of small virtual tapes, allowing them to turn on collocation for all clients without the usual penalty of hundreds of partially used tapes. It would also allow users to have dozens of virtual tape drives to perform reclamation at any time without causing contention for tape resources.
EMC/Legato's NetWorker understands disk, too
In Version 7, EMC/Legato's NetWorker introduced support for simultaneous reads and writes to a file type device. EMC/Legato realized that a disk can obviously read and write at the same time, so they simply needed to allow the application to do that. This allows for some interesting activities, like initiating cloning before a backup is complete.

Ejecting virtual tapes
How you eject virtual tapes will determine whether you require a standalone (see Standalone virtual tape library) or integrated (see Integrated virtual tape library) VTL. As discussed previously, a major advantage of VTLs is that they don't require any changes to your existing backup process or configuration. The one exception is if you don't copy your backup tapes and send the copies offsite. Although it isn't a best practice to do so, many environments eject their original tapes and send them offsite. This works fine with a PTL but, as of this writing, only one VTL (Spectra Logic) supports the ejection of virtual tapes. Therefore, companies that eject their original tapes and wish to use a VTL must usually do one of two things: learn how to copy tape or use an integrated VTL. The approach that's best for your environment will be based on individual preference.

Some observers believe the tape-to-tape copy method with standalone VTLs is the only proper way to create physical tapes from virtual tapes. (Standalone VTLs include those from Diligent Technologies Corp., Quantum and Sepaton Inc.) The tape-to-tape copy method allows the backup software to control the copy process, therefore integrating the copy process into normal reporting procedures. However, there are two challenges. The first is the difficulty related to automating this process. Some backup products require the purchase of an additional license, and some need a custom script for this process.

The second challenge is that many environments don't have enough time and resources to copy their backup tapes quickly enough. For many companies, it's all they can do to get their backups done in time to be picked up by Iron Mountain. If you know how to copy your backup tapes, and have sufficient resources to do so, this won't be an issue.

If the challenge of copying virtual tapes to physical tapes is a concern, you should consider an integrated VTL, such as those offered by Advanced Digital Information Corp. (ADIC), Alacritus Software, EMC Corp., FalconStor Software, Maxxan Systems Inc., Neartek Inc. and Spectra Logic.

An integrated VTL sits between your backup server and PTL. It inventories the PTL and represents its contents as virtual tapes in the VTL. For example, if you have physical tape X01007 in your PTL, virtual tape X01007 will appear in your VTL. Your backup software will then back up to virtual tape X01007. At some user-configurable point, virtual tape X01007 is copied to physical tape X01007. When the backup software tells the VTL to eject virtual tape X01007, physical tape X01007 appears in the PTL's mail slot. An important point is that physical tape X01007 looks just like it would if the backup software had backed up to it directly. The backup software thinks it backed up to and ejected physical tape X01007 and, in the end, that's what it did.

Bar-code matching maintains the consistency between the backup software's media manager and the physical tapes. But you need to remember that this method doesn't result in two copies of the tape. The virtual copy of the tape is deleted when the physical copy is successfully created.

There are, however, some issues with this method. For example, what happens when the copy from the virtual tape to the physical tape fails? If the copy failed because the actual tape is bad, you'll need to remove the tape, swap its bar code to a new tape, put the new tape in the PTL and tell the VTL to try the copy again. (This will only work if your bar codes are removable.) If this happens occasionally, it's not a major disadvantage. But if it happens every day, it becomes disruptive. You also need to realize that this process is happening without the knowledge of the backup software, so if something happens with a tape copy, the VTL will need to notify you of the problem. This results in another reporting interface, which might be considered a disadvantage. Another potential problem arises if the VTL puts more data on the virtual tape than can fit on the physical tape, preventing creation of a physical copy of the tape. Integrated VTL vendors ensure that this doesn't happen by stopping before the normal PEOT. However, standalone vendors might say this practice increases the number of tapes to purchase and handle, and adds to your costs.

This was first published in April 2005

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