What's the true size of an RA92?

What's the true size of an RA92?

[Michael Sokolov]

Hi,

I wonder, does anyone know for sure what is the user capacity of an RA92 in
blocks? I'm now updating disktab(5) and the driver tables in 4.3BSD-Quasijarus
to cover new RA disks, and I can't figure out the user capacity of RA92 in
blocks. For all other RA disks with no exceptions (all RA8x, all RA7x, and
RA90) the user capacity in blocks is exactly equal to the number of cylinders
multiplied by the number of heads multiplied by the number of sectors, i.e., no
funny reserved sectors or tracks or anything like that. Looking in the
disktab(5) from Ultrix V4.2 I see a perfect match between the geometry and the
partition sizes for all disks except RA92. The RA92 entry indicates 3279
cylinders, 13 heads, and 69 sectors per track, but partition c is listed as
2940951 blocks instead of 3279*13*69=2941263 blocks. Does anyone know for sure
whether the user capacity of an RA92 is 2940951 blocks, 2941263 blocks, or
something else altogether?

--
Michael Sokolov
Special Agent
International Free Computing Task Force

ARPA Internet SMTP mail: msokolov at meson.jpsystems.com

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Subject: What's the true size of an RA92?
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For any MSCP disk, the proper way to find out how many sectors there are
is to ask the disk.  The `unit online' command (which has to be used anyway,
to tell the controller to connect to the disk) reports the unit size in
sectors; the `get unit status' command reports the number of sectors per
track, tracks per group, and groups per cylinder.  (The term `group' here
is MSCP-speak which I've never really understood; the idea seems to be that
groups are collections of cylinders that can be switched between with relatively
little time penalty, whereas switching between even adjacent cylinders is more
expensive.)

Beware, however, that modern disks usually don't have a fixed number of
sectors per track; the tracks furthest from the spindle have more sectors,
so that more of the disk surface can be used without too much density
variation.  Such `zoned' disks weren't common (maybe they didn't even exist)
when the MSCP spec was first written; maybe the RA92 is new enough that it
has zones.

I've never been convinced that worrying in great detail about track and
cylinder sizes gains much performance anyway, but that's another story.