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Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Thomas Koenig <tkoenig@netcologne.de> Newsgroups: comp.arch Subject: Re: ancient disks, Architectural implications of locate mode I/O Date: Sun, 7 Jul 2024 06:35:40 -0000 (UTC) Organization: A noiseless patient Spider Lines: 33 Message-ID: <v6dcvs$7shp$1@dont-email.me> References: <v61jeh$k6d$1@gal.iecc.com> <v6anes$3mvnv$1@dont-email.me> <v6bm29$2ev1$1@gal.iecc.com> <v6c56i$3u9qe$1@dont-email.me> <v6cctp$2k8r$1@gal.iecc.com> <v6dc8r$7ra8$1@dont-email.me> Injection-Date: Sun, 07 Jul 2024 08:35:40 +0200 (CEST) Injection-Info: dont-email.me; posting-host="4e89bd446efd77165befcee164bc3155"; logging-data="258617"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19bTFsWPQ0J/cC5yZxbKV+j2WUMoKTXOXA=" User-Agent: slrn/1.0.3 (Linux) Cancel-Lock: sha1:D3zCbDmU40rbWGega0nS+MFnIPQ= Bytes: 2700 Stephen Fuld <SFuld@alumni.cmu.edu.invalid> schrieb: > John Levine wrote: > >> According to Stephen Fuld <SFuld@alumni.cmu.edu.invalid>: >> >> I believe that's what they did with VSAM. >> > >> > Agreed in the sense that VSAM replaced ISAM, but, and I am getting >> > beyond my depth here, I wasn't aware that PDSs used ISAM. I had >> > thought they were a thing unto themselves. Please correct me if I >> > am wrong. In any event, PDSs in their original form lasted beyond >> > the introduction of VSAM, or the PDS search assist functionality >> > wouldn't have been needed. >> >> A PDS had a directory at the front followed by the members. The >> directory had an entry per member with the name, the starting >> location, and optional other stuff. The entries were in order by >> member name, and packed into 256 byte records each of which had a >> hardware key with the name of the last entry in the block. It searched >> the PDS directory with the same kind of channel key search it did for >> ISAM, leading to the performance issues Lynn described. > > > Yes, I don't disagree with any of that. But I got the impression from > your previous posts that IBM had replaced the search key fields of a > PDS with some kind of VSAM (i.e. b-tree or such) varient, as they did > with ISAM. If that is true I had never heard about it. They introduced PDSE, see https://www.ibm.com/docs/en/zos-basic-skills?topic=sets-what-is-pdse It appears they fixed many of the problems with the original design, but not all (why is the number of extents still imited?) It also seems that RECFM=U for load modules is no longer supported, you have to do something different.