I'm not sure I understand the distinction between mainframe and distributed DASD. My guess is that distributed DASD refers to open system SAN capabilities. It could also refer to DASD used in S/390 cluster configurations that are accessed over ESCON or FICON. You will have to ask the vendor's representative what they mean and then determine whether or not they know what they are talking about.
Mainframe DASD access is based on something called "count key data." If you don't know about it already and nobody is telling you to learn it - don't bother. Open systems SAN DASD uses serial SCSI. The two are not even close to being compatible. I'm sure IBM, EMC and HDS all have DASD that is for mainframe use only. The configuration of the I/O ports should be selectable by the customer in most cases.
The best way to think about SANs is: they are the application of storing functions on a network. It is not the DASD, not the switch. It is the transport of storage traffic that defines the SAN. Otherwise, the physical stuff is just a network. This is a terribly confusing area, but I believe everybody would be a lot better off if they thought about both SAN and NAS as network applications. Then it becomes clear that DASD is a type of end node that supports storage functions.
Note: Do not confuse DASD with DAS. DASD is direct access storage device and DAS is direct attached storage.
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This was first published in October 2001