The limit of 16 extents per volume for a non-VSAM data set dates back to the mid-1960s and release 1 of OS/360. A Format 1 DSCB has space for 3 extent descriptors and a pointer to a Format 3 DSCB, which has space for 13 more extent descriptors AND ALSO a pointer to another Format 3 DSCB. Thus the VTOC was architected to allow an infinite number of extents in one data set, as long as the volume has enough free space for one more Format 3 DSCB. For reasons unknown to me now, but probably due to the smallness and expense of storage in the 1960s, IBM chose to cap a data set's VTOC usage to a maximum of two DSCBs for a non-ISAM data set and three for an ISAM data set. The way VSAM data sets are described in terms of DSCBs is that you start with a Format 1, it points to a Format 3, that F3 can then point to another F3, etc. up to however many F3 DSDBs are needed to hold all the extents that are allowed by some other constraint somewhere in the system software. That is the only reason why a VSAM data set is limited to 123 extents. Nine full Format 3 DSCBs can be used with a 10th Format 3 DSCB having only three of its possible 13 extent descriptor slots filled in. Somewhere else in MVS is the limit of 123 imposed. This limit came about when VSAM was invented along with making OS/360 use virtual storage. The system catalog's internal structure was redesigned at the same time to use VSAM in which to store catalog entries. So the limit of 123 extents, I believe, is related to some part of a VSAM catalog's internal structure.
One DEB can have 255 different extent entries in it. They could theoretically all be on the same volume if there weren't the other constraint about 16 extents per volume for one data set. In fact they can be all on the same volume if multiple different data sets, all on one volume, are concatenated. Remember the DEB was invented in the mid-1960s also, long before VSAM came along with the new larger 123 extent limit. I don't know about the limit of 59 volumes per data set. The TIOT's constraint looks like a reasonable explanation. Bill Fairchild Franklin, TN ----- Original Message ----- From: "Lon A CTR USARMY HRC Storr (US)" <[email protected]> To: [email protected] Sent: Thursday, November 21, 2013 1:11:58 PM Subject: Re: Extents limit for HFS (UNCLASSIFIED) Classification: UNCLASSIFIED Caveats: NONE According to my personal notes The restriction of 127 extents per volume (device) comes from the DEB: DEBLNGTH is the number of double-words in the DEB (up to 255 ===> 255 * 8 = 2040; (2040 - 71) / 16 = 123). The restriction of 59 volumes (devices) per DD comes from the TIOT: TIOELNGH is the number of bytes in the TIOT entry (up to 255 ===> (255 - 16) / 4 = 59) Alan -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of DASDBILL2 Sent: Thursday, November 21, 2013 1:45 PM To: [email protected] Subject: Re: Extents limit for HFS I think there was some rational explanation given several years ago. Check the archives, John. Something about how many whatevers could fit within one such-and-such, where both are control blocks within a VSAM catalog structure. I disagree with the other post that mentioned up to five different extents to satisfy the primary size. If this were true, then we wouldn't have a limit of 16 extents for a non-VSAM data set, but rather 12 (16 minus 4). Each of those five extents that might be necessary to fulfill the primary request count towards the total, whether the total is 123 or 16. And, since the primary request could also be fulfilled with only one extent, then there can still be 122 more non-primary extents in a VSAM data set. Bill Fairchild Franklin, TN ----- Original Message ----- From: "John Gilmore" <[email protected]> To: [email protected] Sent: Thursday, November 21, 2013 11:11:46 AM Subject: Re: Extents limit for HFS Why the magic number of 123 extents per volume? 127 is more plausible. What else is going on here? On 11/21/13, John Eells <[email protected]> wrote: > > z/OS DFSMS Using Data Sets, topic 3.9.2.1, "Creating HFS Data Sets": > > ... > > These data sets can expand to as many as 255 extents of DASD space on > multiple volumes (59 volumes maximum with 123 extents per volume). > John Gilmore, Ashland, MA 01721 - USA ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN Classification: UNCLASSIFIED Caveats: NONE ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
