What is the difference between DASD and Tape? I know that in the past, DASD was attributed to Hard Drive storage and Tape was actually magnetic tape. In our shop we are using "Virtual Tape" Which I am pretty sure is actually a hard drive.
The reason I am asking this question, is I have a job what requires a crazy amount of cylinders. We were advised by our storage guys to allocated 4300,4300 and that the system would be able to handle it as long as we released what we didn't use. As it turns out, the system was not able to reserve the cylinders that we required. So I was thinking of maybe using tape for the sequential files. This doesn't seem like a great solution because all of jobs are set up with GDG datasets (which can't be used on tape), so I wanted to try and find out what (if any) issues we would encounter before redesigning.
I am scared that we will run into the same issue trying to allocate a tape data set that big because if virtual tape and DASD are basically the same thing, it would stand to reason that the same error could occur when trying to reserve tape space of that size.
So can some explain to me the difference between DASD and Virtual Tape?
The problem with DASD is that z/OS still thinks it is working with some kind of 3390-device with all its physical characteristics while the real storage-devices have totally different characteristics and far fewer restrictions.
So when you want to allocate 4300 cylinders the system wants to reserve a contiguous are of 4300 cylinders, which is about half of a 3390 model 9 device. So if you have to other datasets of one track each located at some unfavorable locations there might not be a sufficiently large contiguous free area and the allocation will fail.
If you start with an empty 3390 model 9 device you could succesfully allocate your dataset plus one additional extent, but the next extent would already require a multivolume-dataset. But it is unlikely your storage-group has reserved one volume only for your dataset.
The thing with (virtual) tapes is you always start out with a volume for your exculsive use, so there are no other datasets that could interfere with your space-needs.
So while the background-storage might be similar for DASD and virtual tape devices the operating-system imposes different (artificial) restrictions on them to emulate the behaviour of some legacy-devices.