info needed: IPI disk

[dupuy at cs.columbia.edu]

Your Sun salesman was right that the Sun IPI disk is "single-headed" (not
that it has a single read/write head, but that it can only read/write to a
single head at a time).  So you can reasonably assume that the Sun
706A/707A disk is the CDC-97209-12G.  This same disk drive is availbale
from a number of third party peripheral suppliers (Falcon, and National
Peripherals, to name but two).

The CDC-97229-12G "double-headed" is the same physical disk as the 97209,
with modified electronics that support interleaved I/O to pairs of heads.
Since the disk normally has an odd number of data heads (15), some
capacity is lost relative to the 97209 (1154 MB unformatted, rather than
1230MB unformatted).  The disk geometry appears to have half as many
tracks/cylinder (rounded down), but each track is twice as long.  To quote
from the Imprimis blurb:

	The 6MB/sec data rate is attained by reading form and writing to
	two heads simultaneously.  The splitting of the data on a write
	and the combination of the data on a read is done entirely by
	the disk drive and is transparent to the disk controller and/or
	adapter.  The data may be interleaved across the head pair using
	a bit, byte or word interleave scheme.

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