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Disk and tape on the same HBA? Think again.

Don't connect tape and disk to the same HBA in a SAN.

Tape backup systems can use the same Host-Bus Adapters (HBA) as the disk storage system they are backing up. On first blush, connecting disk storage and tape backup drives in this fashion seems like a no-brainer: Think of the cost savings. But the obvious first reaction is often too simplistic. In this case, IBM points out that there are disadvantages as well as the obvious advantage, especially on a fabric-based SAN that allows zoning.

Zoning is used in a SAN for a number of reasons. In its FAStT (Fibre Array Storage Technology Storage Server) Best Practices Guide IBM lists three principal reasons for using zones: administer security, by setting up boundaries between different parts of the SAN; customize your environment by setting up local subnets of the SAN fabric; and optimizing IT resources, by grouping assets logically.

But if you use zones, which is a good idea, then IBM recommends using different HBAs for tape and disk connectivity and creating a separate zone on the SAN for the tape. Among other things, this provides a degree of protection for tape backups if another device performs an LIP (Loop Initialization Process) reset. This can happen if the operating system performs a bus-reset routine. With a separate HBA on a separate zone, the tape backup will not be affected by such problems and will continue normally. Otherwise a reset or similar problem could slow down the backup or cause it to fail altogether.

The Redbook is devoted to the FAStT array, but there is a lot of advice, especially on designing and configuration, which applies generally to fabric based SANs.

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.

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