Date   : Tue, 06 Jul 2004 18:54:58 +0200 (BST)
From   : Johan Heuseveldt <johan@...>
Subject: Re: Acorn Winchester unit

Hi,

On Tue 06 Jul, Mike Tomlinson wrote:
> In article <Marcel-1.53-0705205849-b49xSBG@...>, Johan
> Heuseveldt <johan@...> writes
> 
> >Why? Because SCSI drives won't allow a low level format,
> 
> oh yes they do!  The command's called Format Unit.

Well, this is an official SCSI command, being there from the old
days! (the command number is 4 iirc)

But remember: On a day in the IDE developement, the format command was
still there, but a physical low level format was not done anymore. The
format command still does something in the line of checking/initialising.

I have the strong feeling the same happened with SCSI.

So modern drives, drives of ten years old and even older than that,
cannot be reformatted to 256 bytes per sector. I think a low level
reformat is never possible these days. The reason could be that all
bytes not being data bytes are re-written regulary, so they keep
reliable and don't deteriate, as - once the drive is formatted - they
never get written any more. Also, the drive electronics/controller
use the quality of the electronic signal from the read track to align
the head(s). Therefore drive formatting is done at the factory, and
may never be done again.
I think SCSI has gone the same way as IDE.
Or the other way way around of course!
I cannot see any reason to think/believe otherwise.

Why has it evolved this way? Because track density is so close that
a stepper motor is now hopelessly inadequate, and a controlled solenoid
(coil) is used nowadays.

Along with this, multiplatters are no longer used or as few as
possible - and a single disc is used, which is mechanically preferred.
IDE still supports the standard figures for number of heads, tracks
per platter/disc, sectors per track and bytes per sector. The bytes
per sector remain the same, but the other three can be changed in
whatever scheme you like. It is called a logical model, and the
default is just the model as expected (with most BIOSes)
And, of course, there is LBA too.

Think about it; there is nothing to hang on for the exact physical
location on the platter/disc, when writing a complete track.

I believe both IDE and SCSI have grown up to advanced electronics and
controllers. Perhaps benefit from each other in developement!
(well, most likely, isn't it?)

That's why I said 'generally speaking low level format is no longer
supported in SCSI HDs'.
The exception is old stuff. And there is no doubt a difference between
manufacturers. I would say the less data storage on a drive, gives you
a hint/clue to look for low level formatting supported in its technical
data! And a stepper motor is also a good hint. But I have no idea what
was history first: the stepper motor or low level format ability.

> > and there is no way to use a logical model of 256 bytes sector on
> > top of the physical 512 bytes per sectors /in/ the drive.
> 
> Yes, there is.  Providing the drive's firmware doesn't make assumptions
> and will allow a sector size of 256 to be set.

Would be interesting. But I haven't seen it.
In a book from a few years back, detailing the SCSI bus, there is
nothing like that mentioned.
But hey, developements are quite fast these days/the last years. :-)

If this is really so, we have an alternative: Instead looking for
old SCSI hard discs, we can now look for modern SCSI drives which
support this logical or soft model!

So Mike, give us a few hints and mention a few drives that fit
the bill please!

And now the snag:
There is a limit on the drive's maximum size imposed by ADFS. Free
space Map is using three bytes for block(=sector)numbers. This would mean:

 2^24 sectors/blocks = 2^24 * 256 bytes = 2^32 bytes = 4 GBytes.

Or is ADFS using other measurements when dealing with hard drives?
(but don't think so)

So any modern drive is a waste of space, as 4 Gigs is pretty low
these days. So:

 1 Is a not-too-new drive of 4 GB - or just a little more - still young
    enough to support the /logical/ model of 256 bytes per sector?

 2 Or can we find some of the latest drives that still support the low
    level formatting, including 256 bytes per sector?

> The drives Acorn used in the FileStore E01S (there were, IIRC, E20S and
> E40S external disk units) were Rodime SCSI drives.

In my SJ Research's MDFS there is such a drive too.
Perhaps also using 256 bytes per sector?

Perhaps you can check the original info on these Rodime drives. Most
likely Acorn choosed drives that /allowed/:
 1 low level formatting,
 2 with different sector sizes,
 3 including 256 bytes per sector.

I did some tests years back, and low level format allways
failed. Another person did that too, with many more drives to
test.
It was he who found out about the Segate ST157N and Syquest 44MByte
removables.

No doubt there are more. But finding them is a complete different
chapter in our book. In two ways:
 1 Someone has to find such a drive, which can be used
    (checking data sheets, /and/ testing such a drive)
 2 That drive has still to be available in some numbers (obviously)

> > 1: Try to find the Seagate SCSI drive: ST157N (the 'N' is vital!)
> 
> The N is Seagate's way of indicating the drive is SCSI.

Yep! :-)
A drive without the 'N' also exists; the IDE version.


I hope I have better clarified my views now?


Johan

-- 
Johan Heuseveldt <johan@...              >
  aka  waarland

  The best place is a Riscy place
 
The cost of living hasn't affected its popularity.