On 6 February 2017 at 09:22, R.S. <r.skoru...@bremultibank.com.pl> wrote:
> W dniu 2017-02-06 o 14:59, Ron Burr pisze: > >> As far as I know, reading a physical sequential (or partitioned dataset >> member) in reverse order can only be done using BSAM (via the BSP macro). >> Not that many applications require that mode of processing. But if one >> does, well then, that appears to be the only option. Mind you, there are >> some restrictions inherent in using BSP, as the manual explains. >> The COBOL manual states that you can process a QSAM tape dataset in >> reverse order by doing an OPEN REVERSED, but I suspect that the dataset >> will actually be processed using BSAM, not QSAM. >> > > It's worth to mention that performance of reverse processing dataset on > real tape is worse than horrible. > And IMHO it's not argument for using virtual tapes but for not using > datasets on tape for applications. Tapes are for backup and ML2. > Are you two perhaps mixing reading in "reverse order" of records with "read backward"? Read Backward is a CCW that (on old reel-to-reel tapes, at least) moves the tape backwards past the read head and transfers data into main storage in byte-by-byte decreasing address order. So the data ends up in normal order in storage, but if your app knows somehow that the tape is positioned at the end of the data, it doesn't have to rewind or backspace and then read forward. I have no idea if any modern real or virtual tape supports this, or if COBOL or even BSAM/QSAM ever did. Tony H. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
