[rescue] SCSI Replacements

[Dave McGuire]

On 6/28/24 10:27, Mark Benson via rescue wrote:
> I refer to them as “Emulators” because they are not a raw SCSI device.

   What, exactly, is a "raw SCSI device"?  The fact that it consists of 
multiple internal components, one of which is removable, couldn't 
possibly make that distinction.  I can go pull the logic board off of 
the HDA of a Seagate ST15150N too.  That doesn't make the assembled 
device anything other than a SCSI drive.

> While they do work to all intents and purposes like a real SCSI block device the modern versions “virtualise” the block device images,

   All SCSI drives do exactly this internally.  Block number 213 is not 
guaranteed to physically reside at byte offset 213*512 on a SCSI drive, 
and if it does, it's not guaranteed to remain there throughout the 
service life of the drive.

> allow up to 6 devices from a single card and even support hot-swap if all the LUNs are unmounted.

   The Emulex MD21 is a SCSI<->ESDI bridge.  Ubiquitous in the early 
1990s in higher-end systems, these were never referred to as 
"emulators".  Each connects to one OR TWO ESDI hard drives and presents 
one or two LUNs to the host.  While it was not designed to allow 
hot-swapping of their connected drives, I have hot-swapped them.

>  This to me puts this function of the device more into Emulator territory as the controller is doing more than just pretending to be one device.

   The aforementioned Emulex MD21 is a controller that can be more than 
one device.  It can even format its drives.  No one in the industry ever 
called it an emulator.  Emulex themselves called it a "Disk Controller". 
  So the difference is the upstream host interface is SCSI rather than, 
say, ISA or Sbus?

   Sorry, still not buying it.  These are "SCSI drives" like any other. 
The fact that they didn't come from Seagate or Fujitsu or the fact that 
their "HDAs" are removable, have a documented interface, and can also be 
plugged into (say) a camera doesn't change that.

   What made a SCSI drive a SCSI drive was always a relatively loose 
definition.  This stems from the simple fact that SCSI is a high-level 
interface with several layers of abstraction between what the host sees 
and what is recorded on the media, which doesn't have to be magnetic, 
and indeed doesn't have to be round.  This has been the case since 
SCSI's inception in the beginning of the 1980s.

               -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA