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When to mix SCSI devices and busses

Mixing SCSI device and bus types may lower performance, but there are times when it is acceptable.

Over the years there have been no fewer than 16 different kinds of SCSI drives and three bus configurations (50-, 68- and 80-pin). In an ideal world, one would not mix different kinds of devices and busses on the same SCSI chain. But storage managers know the world isn't an ideal place. Although mixing device and bus types may pull the SCSI system down to the speed of the slowest component, there are times it can be done without lowering performance.

The situations that crop up most often involve mixing 68- and 80-pin busses and devices, or else mixing different kinds of SCSI modes on 80-pin busses. (For the most part, 50-pin busses and devices are obsolete in enterprise installations.)

Situation 1: Connecting Wide (68-pin) drives to a Wide (68-pin) bus. A Wide bus can run in three different modes: single-ended High Voltage Differential (HVD) and Low Voltage Differential (LVD). You cannot mix HVD and LVD devices on the same bus without damaging something. Fortunately HVD devices are increasingly rare on Wide busses. You can mix LVD and single-ended devices on the same Wide bus, but the combination will run at the slower single-ended speed.

Situation 2: Connecting Wide (80-pin) SCA-2 (single-coding attachment) devices to a Wide (80-pin) bus. As in the above case, the bus can run in single-ended HVD or LVD modes. You can mix LVD and single-ended drives on an 80-pin bus, but, again, the bus will revert to the slower, single-ended speed. However, in this instance, the mode of operation will be determined by the DIFSENS level on the bus.

Since there is no term power line on the 80-pin connector, termination must be provided by the adapter or the backplane. Some older backplanes will support only single-ended mode because of the way they handle signal grounding.

This and several other cases are covered in an online tutorial.

If a problem does arise from mixing SCSI devices, the most likely symptoms are:

  • the server refuses to recognize the device;
  • the drive will not spin up; or
  • performance is seriously degraded.

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 20 years he has been a freelance writer specializing in issues related to storage and storage management.

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