Feature

How disk has changed backup

Ezine

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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 supports hardware compression that doesn't impact performance.

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SHOULD A VIRTUAL TAPE LIBRARY BE SHARED?
Partitioning makes it possible to share a 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.
Ejecting virtual tapes
How you eject virtual tapes will determine whether you require a standalone or integrated 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 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, 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, the VTL internally copies virtual tape X01007 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 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 work only 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.

This was first published in November 2005

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