Most companies buy mirrored tape controllers to do exactly what the name implies -- make a mirrored copy of one or more tapes during the backup process. However, most of the products on the market can do other things as well. It's worth taking a look at what your mirrored controller can do to see if it can help you in other ways.
The feature sets in mirrored tape controllers vary from model to model. However some of the more common ones include offline copying and verifying, auto-switching and remote copying.
Cascading allows the controller to automatically span two or more tapes when the data being backed up becomes too large for a single tape. The controller fills the tape in the first drive and then automatically switches to the next one. Of course, you can't cascade and mirror at the same time.
Offline copying and verifying is probably the most commonly used of the additional features. It allows the system to copy the tape in one drive to the tape in another. The verification feature checks each tape against the other. Not all controllers that support copying also support verification.
Some controllers contain the hardware and firmware, notably buffering, to provide remote and local backups simultaneously. The controller backs up the data to a local drive and sends the same data to a drive that can be clear across the country.
Mirrored controllers that support tape libraries can do even more. One of the features supported by these products is making multiple simultaneous copies of tapes or tape sets, often up to the number of drives in the library.
Of course most mirrored tape controllers support striping to increase throughput by spreading the data across multiple tapes.
Dual mirrored controllers, which support two tape drives, include products from Tapelabs and GST. Unylogix offers controllers that support mirroring of tape libraries.
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Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years, he has been a freelance writer specializing in storage and other computer issues.
This was first published in August 2004