Just out of curiosity, why multiple passes. I know that they must have
their reasons... Something like residual data on the edges of the track
or some such, since there is a possibility of the track not being
recorded in the exact location of the previous write?
Bill Fairchild wrote:
I assume you are using QSAM on the 2 and 3 blocks per track pass to make the
ALC program easier to write, but a single full-track block can be written
with EXCP if you want to go there - takes a little more ALC programming. Chain
enough full-track writes together to do one whole cylinder per I/O request.
Then you lose minimal time due to extra rotations when switching from one
track to the next.
BIG CAVEAT: make sure there are no data sets in use on the volume before
you start erasing it. Especially system data sets. Especially JES checkpoint
and paging data sets. Been there, done that, got major ugly scars (master
console went SOLID red with undeletable critical error msgs just before system
crashed bigtime). ENQ is a good way to test for most data sets.
High-speed erase of huge number of volumes (such as in a DR facility after
testing one's DR plan) is a difficult problem, and very necessary. Whoever
comes into the DR shop after you leave can see all your data unless you erase it
when you are finished with your test. And that includes paging data sets,
which also need to be erased, but you really ought to do them last. A
non-MVS standalone process is probably best (as in New Era). How to erase is not
itself difficult, but how to erase super-fast a huge number of volumes becomes
the problem. One way to speed it up is to ask different DASD vendors for
their proprietary full-volume erase command. STK has one, and doing one I/O
request erases the whole volume. Other vendors may require doing full-track
writes to tens of thousands of tracks, a very time-consuming process. And
DoD/IC [1] standards require multiple passes with special bit patterns.
If it takes you one hour to download all your data at the beginning of a DR
test shot, plan on taking at least one hour to erase everything. Test it all
at home before you go to the DR place, too. Then you can accurately plan on
how long it will take to erase at the DR place.
Bill Fairchild
[1] Department of Defense/Intelligence Community
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