Every MVS volume I have seen in the last two decades is on an emulated
3390 drive, although no doubt somewhere people are still running real
3380s or 3390s. From MVS's viewpoint, it thinks every DASD unit
address is a physical DASD drive even though the DASD Subsystem is only
emulating the architecture of a real 3390 drive and 3990 controller, In
the context of z/OS and modern DASD, an "emulated drive" and an "MVS
volume" should be acceptable alternatives for referencing the same
entity, a DASD drive and its contents (although technically, the "drive"
is the container and the "MVS volume" the content). An actual backend
DASD drive in the DASD subsystem would be a physical drive, not an
emulated drive as seen by z/OS.
Re comment by R.S.:
I agree that one is extremely unlikely to lose a single MVS
volume/emulated drive in today's DASD subsystems from HW failure. but if
there is any way a single volume can be accidentally forced offline,
hung, undefined, or trashed out from under a running MVS system, I'm
sure a System Programmer somewhere will discover it. If all else
"fails", unintentionally sharing DASD and unintentionally writing to the
wrong unit address and volume from an independent system can do anything.
Joel C. Ewing
On 12/04/2014 12:46 PM, Bob Shannon wrote:
>> With current emulated DASD and PAVs, performance is probably no longer an
>> issue, but I believe multiple page data sets on one volume is still a
>> potential availability issue: You wouldn't >want failure of a single
>> emulated drive to compromise two different systems at the same time, and I
>> seem to recall it used to be fatal to have failure of multiple page data
>> sets on the same >system at the same time.
>
> You seem to have intermixed mixed "volume" and "emulated drive". Unless the
> recommendation has changed, there should only be one page dataset per MVS
> volume. IIRC MVS remembers the last head position and performance suffers
> when the head has moved. If you are considering the backend SCSI drives used
> when emulating MVS volumes, they are in a RAID array which is designed to
> tolerate SCSI failures. I don't pay any attention to them.
>
> It may be time to revisit old paging ROTs. Does anyone have a double or
> triple digit paging rate anymore? Is the 30% rule still valid? (We completely
> ignore it). Does zFlash obviate the old ROTs?
>
> Bob Shannon
> Rocket Software
...
--
Joel C. Ewing, Bentonville, AR [email protected]
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