Formatting CDC Wren drives

[Spencer Garrett]

In article <818 at stca77.stc.oz>, peter at stca77.stc.oz (Peter Jeremy) writes:
> Does anyone know how to format a CDC Wren IV or Wren V drive under SunOS
> with other than 512 byte sectors?  From my reading of the Wren IV manual,
> you gain ~10% space by using 1K sectors (and possible still more with
> 2K, 4K or 8K sectors).  I can't find anything in any of the Sun manuals
> describing this, but believe it should be possible.

Don't assume you'll always get more storage by using larger sectors.  It
just depends on how they happen to pack onto the tracks.  I've seen cases
where shifting from 512 to 1024 bytes resulted in less usable storage per
track.  This is complicated by the fact that most high- density SCSI
drives use variable-density recording, so the packing will not even be
constant across all cylinders of the disk.

I can't give you a full format.dat entry for the Wren V since I've always
formatted mine using diag, but I can give definitive numbers for its size.
A "read capacity" command reports exactly 1173930 sectors (512 bytes each)
which is 2 * 3 * 5 * 109 * 359.  Since the number of sectors per track
isn't constant you can't use an actual number to build a file system.  The
only way to utilize all of the sectors is to partition the list of prime
factors between c/h/s and ignore the physical parameters of the disk.  I
use

	716+2	cylinders		(2 * 359)
	15	heads			(3 * 5)
	109	sectors per cylinder	(109)

This works swimmingly, and allows me to use every available byte on the
disk.