------------------------------------------<snip>---------------------------------
Perhaps there is more to the emulation of CKD than the thread touches
on. Disk Drives stopped recording in CYLS some time ago because the time
for head switching is greater than the minimum seek time. Drives today
record in a serpentine method across the platters, doing a switch-back
(best word I can think of) to the next head at intervals defined by the
HDD designers.
The whole idea of tracks and CYLS is really dead and buried as far as
the real hardware is concerned.
------------------------------------------<unsnip>----------------------------------
Long ago, when I worked in a heavily-modified CP67/CMS environment (on
168's and 2305's), we had a fairly sophisticated mechanism for handling
page files on the 2305's. Each track was written with three page
records, and a "gap" record between each pair of page records. We
learned that the "gap" record afforded us enough time to swap exposures
on the 2305, so we could read three pages, from the same track, on three
different exposures of the 2305, in one revolution. We did the writing
on a different trio of exposures as well. (Thank you, Grant Tegtmeier.)
I would venture to guess that modern DASD storage uses a far more
sophisticated version of the same mechanism to read/write records on
multiple physical drives today, even though the "logical" perception,
from the program point of view is the "traditional" view.
Back to the original starting point of this thread: I, for one, am
grateful for the "solidification" of a single geometry. Recomputing
space parameters and updating legacy applications can become a MAJOR and
EXPENSIVE process, frought with errors and omissions. By maintaining the
same geometry, we can let these old applications, etc. die off by
attrition, rather than having to spend valuable resources updating
legacy code and JCL because a "new device" is being brought into the shop.
The fact that a device has more "cylinders" available should NOT be a
matter for application programmers to be concerned about; let the system
manage the space, and the systems programmers worry about the details,
if they really feel it's necessary. Same opinion concerning "tracks". By
leaving the basic geometry alone, a shop can go forward with new
development and let the programmers learn about SMS-friendly space requests.
My two cents worth. :-)
--
Rick
--
Remember that if you’re not the lead dog, the view never changes.
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