Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!rutgers!cmcl2!phri!roy From: roy@phri.nyu.edu (Roy Smith) Newsgroups: comp.periphs Subject: Re: External Sorting Keywords: Sorting, Tapes Message-ID: <1991Jan28.220933.16242@phri.nyu.edu> Date: 28 Jan 91 22:09:33 GMT References: <1991Jan28.031017.19886@comp.vuw.ac.nz> <1991Jan28.181354.11095@iecc.cambridge.ma.us> Sender: news@phri.nyu.edu (News System) Organization: Public Health Research Institute, New York City Lines: 30 johnl@iecc.cambridge.ma.us (John R. Levine) writes: > under MVS you can start read operations on several files, then process > the records in whatever order the physical reads happen to finish. How is this any different from what you can do in BSD systems by issuing several non-blocking reads and then select(2)ing whichever one finishes first? I think the real question is not so much whether the software supports the operation but whether the I/O system is macho enough to keep up with 8 (or more) 6250 drives, all running at 100+ ips. Keep in mind that what passes for a smart device controller on most Unix boxes today (say, a Ciprico Rimfire disk controller for a Sun) pales in comparison to the sorts of things you find on IBM big iron. > Mainframe tape drives have always had the ability to read tapes > backwards, since an oscillating sort that merges alternate passes forward > and backwards avoids time-consuming rewinds between passes. My trusty 1975 pdp-11 peripherals handbook indicates that the TU-16 (TJU-16?) could do reverse reads. Was there ever a version of Unix which supported that? I could see doing it either by adding a special ioctl, or by adopting the convention that a seek backwards of a single block on a tape device would be cached, and if immediately followed by a read of a single block, the two would be folded into a single read-reverse operation. -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy "Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!"