🚀 go-pugleaf

In article <m0mO0oC-000098C@daver.bungi.com>, dlr@DAVER.BUNGI.COM (Dave Rand) writes:
> 1) SCSI devices will always transfer in block unit sizes (usually 512 bytes)
> before requiring service via an interrupt. Even during disconnect/reconnect,
> it *seems* that this is the case, but is not required by the standard. A
> duff's device, with size of 32 (128 bytes), seems to offer the best
> compromise between i-cache usage and speed. Your results may vary. Void
> where prohibited.  Up to 512 could be used, but results are uncertain.

I do some contract work for a company called Ciprico, which manufacturers
high-performance disk, tape, and SCSI controllers for open standard buses.
They have lots of experience with LOTS of SCSI devices.  Let me assure
you that anything that is allowed by the standard, by intent or by
loophole, happens.

Disks are often well-behaved in regard to when they disconnect, but not
always.  Cacheing disks, in particular, may disconnect at any arbitrary
point in a transfer because its internal scheduler has detected that
a desired sector for reading or writing has come up, and the disk needs
to service its internal cache.  Fun, right? Really, REALLY high-performance
cacheing drives that can keep up a SCSI transfer to/from the cache while
doing head-to-cache work MAY become available, but I haven't seen one yet.

Also, tape devices get weird.  For one thing, you could be reading
any arbitrary block size from a 9-track tape, particularly foreign
host tapes (not that a lot of you are going to use 9-track...) BUT
there are DAT and 8mm devices which have ODD SIZED write buffers,
like 250 (NOT 256) bytes.  They suck up that much data, disconnect,
get the tape moving, then reconnect and suck in more data.  Really
sweet, right?

So, I don't know if any of these concerns apply to the current situation,
but felt you ought to know.
-- 
		   John Lind, Starfire Consulting Services
E-mail: john@starfire.MN.ORG		USnail: PO Box 17247, Mpls MN  55417