A tape library consists of one or more tape drives, a storage area for tapes and a robot arm to load and unload tapes from the drives. Each drive in the library has its own connection to the server, usually a small computer system interface (SCSI). The arm is also controlled via SCSI, but does the arm need a separate SCSI controller or can it be piggybacked on one of the tape drives' SCSI controllers?
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It used to be that backup administrators had no choice. Libraries came with the arm hard-wired either to share a controller with a drive or for a separate controller, depending on the make and model of library. However, more and more tape libraries allow the administrator to choose at configuration time.
There are two schools of thought on this. Putting the arm on the same controller as a tape drive means that the arm's bandwidth is stolen from the backup data bandwidth to the tape drive. For that reason Jody Leber, the author of Windows NT Backup & Restore (O'Reilly Publishers, 1998, ISBN 1-56592-272-7) recommends a separate controller for the arm. Others, such as Compaq, don't agree. In discussing its tape libraries at www.compaq.com/products/quickspecs/11336_na/11336_na.pdf, Compaq says that the bandwidth needed to control the arm is so small that it doesn't significantly slow backups.
This is somewhat influenced by data architecture and the way the data is backed up. A full, streaming backup will minimize the commands to the robot arm. A file-by-file backup with files scattered across many tapes will require more. It's probably a good idea to test each setup in your environment to see which gives you the results you're looking for.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.