Storage Management Strategies for the CIO
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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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