The precise location of recorded bits around the track moves around
somewhat. That is why the controller does something called "read with offset" when
re-reading a block that produced a data check (aka "head shaking"). The
read-write mechanism/transducer is moved a tiny distance to the left of the center
of where the track is supposed to be, the data is read, if still bad then
the transducer is moved a tiny distance to the right of the center... If the
data cannot be read correctly (i.e., no data check indication) and you have
moved the transducer so far to either side that it is now within the tolerance
for the adjacent track, then you give up on reading that block.
Another issue is residual magnetism. When you magnetize a substance, a
large % of the molecules are aligned in a certain way. When you erase that data,
not all the molecules get realigned back to their original direction. This
is why skilled recording experts with expensive, sensitive equipment can
recover recorded data that was over-written or erased. The cost of such recovery
is high. The cost of erasing data to the point that it cannot be recovered
by an expert is also high. The trade-off is how valuable is the data to you
and what would it cost you if someone else got the data.
Don't forget that none of the above can be done from the mainframe. To
do these actions, someone must remove the FBA disks from the mainframe
disk subsystem, attach them to a SCSI/Fiber channel with an appropriate
PC or equipment, and then execute the proper diagnostic command to try
and read the residual data. What you will get is the data that the
mainframe CU emulator wrote on the FBA disk, including emulated count
fields and such, which varys by manufacturer. Just makes the job of
someone trying to recover your data more difficult.
From the mainframe CCW point-of-view, once you write over a track, the
previous contents are no longer recoverable.
Bruce Black
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