mjacob@wonky.Sun.COM (Matt Jacob) (02/21/90)
The Emulex MD21 is a good case in point where this actually works fairly well. The MD21 is a SCSI to ESDI controller, and it allows two ESDI drives to be connected (luns 0 && 1). I've found that this configuration works quite well handling both drives simultaneously. Oddly enough, the little 8031 that drives the MD21 doesn't seem to run out of bandwidth in handling two luns simultaneously- I guess that this is really just a overlapped seek time win- the MD21 has only a 16k buffer, so it probably doesn't do any simultaneous data xfers off the disks. The SCSI spec seemed to be pretty unambiguous about this, but this situation does bring to mind a similar ambiguity I ran into: in a multi-initiator environment, how well does a target handle commands from separate initators? I tried this with a couple of Sun's and a CDC WREN IV, and the WREN IV got totally confused as to which request was supposed to get which data! It seems that the target should either return a busy status or accept the commands in sequence (unless it is SCSI-2 compliant which is a different kettle of fish entirely). At any rate, the 1986 SCSI spec (SCSI-1) didn't make this clear, and I found that in a multi-initiator environment that running w/o disconnecting or using reserve/release was the only way to go. -matt jacogb Sun Microsystems