In a message dated 7/8/2005 10:08:36 A.M. Central Daylight Time, [EMAIL PROTECTED] writes:
Bill Fairchild <[EMAIL PROTECTED]> wrote: > VSAM's being obsolete is like saying TSO is obsolete. VSAM has been > the strategic access method of IBM since the mid-1970s. It > underlies many other structures that programmers also don't see, > such as page data sets and catalogs, without which z/OS will not > run very well or long. A better term might be "non-obvious". Page dataset I/O uses ASM's I/O driver, which adds new channel commands onto the end of "seldom ending channel programs" (SECP). MVS/XA added some extensions that allow suspension and resumption of channel programs. Originally, a new channel program for a page dataset volume forced ASM to end its own channel program (one outstanding channel program per subchannel). I'm not sure where that restriction fits into multiple channel programs per volume. The role "The Mysterious Media Manager" plays in VSAM and VSAM- like datasets... What I was thinking when I said VSAM underlies page data sets was that one uses IDCAMS to create a new page data set via DEFINE PAGESPACE, and that what you get after the define is finished is the same as a VSAM linear data set with a large number of empty 4K blocks. I regret implying that standard VSAM channel programs are used to access these 4K blocks. A page data set also looks like a QSAM/BSAM/BDAM/ESDS file, in that all blocks have the same key length (0) and data length (4K), but certainly the individual blocks are read/written by radically different techniques. Media Manager has become the access method used in all new strategic products, such as DB2, IMS fast path, et al. Media Manager accesses data sets that have been defined as VSAM objects but uses different kinds of channel programs to access them than does standard VSAM. And when MM builds its IOSBs before issuing the STARTIO macro, it puts VSAM's Driver ID (X'03') into the IOSB driver ID field. VSAM's data structures are under the covers in many places, but not its channel programs. VSAM started out being THE strategic new access method, but QUASI-VSAM is probably a better term for what is now strategic. Bill Fairchild ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
