🚀 go-pugleaf

In article <ROBM.92Sep4154132@ataraxia.Berkeley.EDU>, robm@ataraxia.Berkeley.EDU (Rob McNicholas) writes:
>I have a DECstation 5000/125 with one on-board SCSI bus (of course)
>and one TurboChannel SCSI bus (PMAZ-AA).  I've heard a rumor that the
>on-board bus uses DMA, but the TC one doesn't.  Is this true?

    The 5000/125 does have a limited DMA capability.  The DMAs are limited
to a "PAGE" at a time and also have a 8/4 byte boundry requirement.
The CPU is required to service the DMA engine, however it does not have
to be involved with every byte transfer as in the PMAZ-AA.

>If this is true, how do people recommend I configure my devices?  I
>have 3 RZ57s (pretty heavy use during the day), .....................

    The old adage of spliting heavy spindle use across busses is still
valid.  Of the 3 '57s put one on the TurboChannel SCSI bus and the other
two on the mother board SCSI.

>.............................................., one RRD42 (hardly ever
>used), 3 8mm EXB-8200s (one is used heavily at night for backups, the
>others used for occasionally making tape copies, and to serve as
>spares in an emergency), .........................................

    When running at night, load - is not all that important, I would
still work on "splitting the spindles" and place the heavy backup one
on the TurboChannel SCSI bus with the one '57.

>......................., and one TZK50 (only used when DEC ships us
>something on TK50).  Should I put all the tapes on the non-DMA bus?  I
>remember that back when I had a 3100 (no DMA), any tape I/O would
>bring the system to its knees, so I'm not sure if this is the right
>thing to do.

    I would not put the '50 on your "system" bus.  The system bus is
the SCSI bus that contains your system disk and the most used user disk.
The other tapes I would also keep off the system bus, but with the low
load on them it is not that important.

-- 
John A. Gallant                                jag@zk3.dec.com
Software Engineer - OSF Engineering Group
Digital Equipment Corp. (603) 881-2472

        In the common people there is no wisdom, no penetration, no
        power of judgment.
    Marcus Cicero