Date   : Fri, 10 Aug 1990 07:21:54 GMT
From   : snorkelwacker!usc!sdd.hp.com!mips!prls!philabs!linus!nixbur!nixpbe!peun11!josef@tut.cis.ohio-state.edu (Moellers)
Subject: How to speed up Ampro LB+ SCSI?

In <12835@mentor.cc.purdue.edu> wilker@descartes.math.purdue.edu (Clarence
Wilkerson) writes:

>Could you implement "scsi device to scsi device" transfer without having
>to go through
>the CPU? This is possible under some circumstances ( e.g. two disks on
>same controller ),
>but I'm not sure of the generality.

From what I know about SCSI, I'd say it depends. (This is standard
answer #75534)

SCSI distinguishes between initiator and target.
The initiator selects a target and then the target requests from the
initiator whatever information is needed (command block, data, message)
or sends to the initiator whatever information it holds (data, status,
message).
Usually, hosts are initiators and devices are targets.
So, in order to do a "device to device" transfer, You'll have to have
one device that can act as an initiator, communication with another
device that continues to behave as a target.
Some tape drives can do this. You just tell'em to read n blocks of data
from target x and then leave it to do it's task. If You were to look at
the SCSI bus, You'd see the tape drive selecting the disk, the drive
requesting command blocks from the tape, then sending data to the tape,
etc.
Probably one or the other controller can do a disk-to-disk-copy locally,
but that would be very controller specific.

--
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