tape sizes

[utzoo!decvax!ucbvax!unix-wizards]

>From decvax!duke!unc!smb at Berkeley Wed Nov 11 16:06:48 1981
The minimum record size is used to distinguish between data and noise
records.  It *is* legal to have records shorter than that, but the
drive is entitled to disregard them if any errors occur.  (As an aside,
I would note that IBM tape drives and controllers are *not* limited to
32K records; the actual limits are somewhat more complex.  First, and
probably most important, the operating system enforces a limit of just
under 32K -- 32760 or some such.  This applies only to folks who use
the standard OS access methods.  Anyone willing to do some fairly
simple (if you ignore error recovery) "CCW"-level programming, i.e.,
pretty close to the nitty-gritty hardware level (this *is* permitted by
OS) can get 64K records.  By using somewhat more complex techniques, it
is possible to read larger blocks than that; in fact, it's even
possible to read arbitrarily large blocks by dynamically modifying the
CCW chain.  I'm not sure if this latter can be done under MVS -- the
virtual memory version of OS/MT -- but it's certainly possible under
the older MVT versions.)