Darren New <dne### [at] sanrrcom> wrote:
> On 6/10/2011 10:52, Alain wrote:
> > It also see a sectors, cylinders and heads count that don't have
> > anything to do with the actual values.
>
> Yep. That's what I said. Of course the drive knows which sectors are where.
> THe interfaces might not, and the OS can't see it, and that's exactly one
> reason why people still use mainframes where important information like this
> isn't hidden from the OS.
>
> --
> Darren New, San Diego CA, USA (PST)
>    "Coding without comments is like
>     driving without turn signals."

That's not quite true anymore, at least with IBM mainframes. That was true with
the early IBM drives like the 3330, 3350, 3380, and 3390 drives.  But the last
several generations of IBM disk drives have been raid devices.  The legacy 3390
disk drive format is emulated by heavily cached controllers, backed by hi-speed
scsi drives.  What appears to the mainframe as a cylinder is really data blocks
spread across several scsi drives.  The model ds8100 I work with emulates about
1200 3390 virtual drives with a total capacity of 15TB.  The actual data is
contained on a hundred or so 10000 RPM scsi drives.  All the time I once spent
trying to optimize disk performance by placing system datasets at specific
locations on the drives to minimize head movement is no longer necessary.

Isaac.

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