But even if they do that under the covers at the track level, I would
expect the DS8K boxes to still allocate/reserve total physical space for
each emulated DASD unit under the worst case assumption that every track
might be fully utilized. Unless advertised as a more restricted version
of 3390, It would be a poor emulation from a mainframer's viewpoint if
your DASD subsystem could have a write to a virtual drive fail from lack
of physical backup storage because someone actually figured out a useful
application that would load up all drives with one max full-track record
per track as permitted by the architecture definition.
The old and long-withdrawn IBM 9393 RAMAC Virtual Array storage
subsystem was was an interesting beast that didn't have fixed track
mapping -- every new emulated track written went to a different place in
the backstore with previous version of the track deleted. There was
even handshaking between MVS and the RAMAC subsystem when a dataset was
deleted so that emulated tracks associated a deleted dataset could be
deleted from the backstore even before the track was re-used. Very
efficient on physical subsystem storage, but also had some peculiar
performance characteristics -- multitrack sequential writes could be
faster than multi-track sequential reads -- And it was possible to get
special alerts indicating the backstore occupancy was getting too high.
Joel C Ewing
On 11/24/22 09:46, Paul Gorlinsky wrote:
Switching from "real" 3380s & 3390s to the emulated 3380s & 3390s has been an
evolutionary path.
For example, with the P370 and its derivatives, each DASD unit was implemented
as a single file on the hosting PC; AWSDISK. Hercules implemented the same file
structure and later added a compressed version with track updates stored in a
new file that could be later reorganized back into the base.
I would guess that the DS8K boxes today got a lot smarter and implemented the track data
or even track record data as separate objects managed by a volume TOC mechanism; which
facilities all the DS8K enhancements of volume cloning, fast copy, check pointing,
versioning, etc. "Sparse type" file systems are also more common today.
So individual dataset space is not a big issue. You have to think at the
enterprise level.
However, the individual still needs to allocate their datasets correctly to
satisfy the OS ... or for the zOS people, get an X37 abend processor...
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