Pros and cons of VTLs

Important VTL features
There are a number of differences among the major VTLs. Some (Alacritus, Diligent, FalconStor) are software only, so you can buy the software and run it on a regular disk array. Other VTL vendors (Maxxan, Neartek) sell a VTL head, which is analogous to a filer head. You use their software and head, but supply your own disk. Finally, some VTL vendors (ADIC, EMC, Quantum, Sepaton and Spectra Logic) offer an entire solution: software, head and disk. Software-only and filer head vendors allow you to redeploy an existing array, reducing your cost. Turnkey products cost more, but have the fewest integration issues.

Most VTLs offer replication or cascading, which replicates one VTL's backups to another VTL. But the tapes in the second VTL won't be considered duplicates by your backup software because they'll have the same bar codes as the original tapes. Also, remember that you'll probably be replicating the entire backup, and most backups aren't block level. Even incremental backups take up roughly 1% to 5% of the amount of data being backed up. This means you'll need to replicate 1% to 5% of your data center every night--a significant undertaking for many environments. Therefore, it may only be possible to use this feature within a campus, as opposed

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to including data from remote sites.

Some VTL vendors are beginning to offer a feature where their VTLs will examine the incremental backup, identify the changed blocks within that backup and replicate only the changed blocks. When that functionality becomes more widely available, replication between data centers will be much easier to accomplish. Today, incremental backup is offered by Alacritus- and FalconStor-based VTLs.

If you have a heterogeneous environment with mainframe, AS/400 and open systems, you might consider a VTL that supports all three environments. Only Neartek currently offers this functionality.

A few integrated VTLs (FalconStor and Neartek) offer a feature called stacking. Stacking copies multiple virtual tapes onto one physical tape, a feature borrowed from mainframe virtual tape systems (VTS). Stacking was important to mainframes because applications were unable to append to a tape. The VTS would present hundreds of small virtual tapes to the app and then stack those virtual tapes onto one physical tape, significantly cutting media costs.

However, the value of stacking in most open-systems environments is questionable because any decent backup product can append to a tape until it's full. But you should be aware that the use of stacking breaks the relationship between the backup software's media manager and the physical tape. And products that support stacking must read the entire stacked tape to read just one of the virtual tapes included on that tape. This feature is only useful if you gain a benefit akin to that achieved in the mainframe environment.

You also need to think about which type of notification the VTL supports, especially if you're considering an integrated VTL. Some support SNMP traps, a few support e-mail notification, while others require you to log into a Web page to be notified of any issues.

If high-end performance is important, you should look for a VTL with a multiple data-mover architecture. Most VTLs run all software on one VTL head. Some vendors use the VTL head as a control mechanism, while passing the movement of the data on to one or more data movers. Need more performance? Simply purchase more data movers. This allows scaling to a much higher level without having to add and administer another VTL (Alacritus, Neartek and Sepaton use this approach).

Finally, remember that VTLs don't perform at the same level, so it's important to conduct performance testing in your environment.

This was first published in April 2005

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