What's the true size of an RA92?

norman at nose.cita.utoronto.ca norman at nose.cita.utoronto.ca
Sun Sep 5 10:40:15 AEST 1999

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

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.

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