[Default] On 6 Feb 2017 11:46:22 -0800, in bit.listserv.ibm-main t...@harminc.net (Tony Harminc) wrote:
>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. IBM 360/370/390/Enterprise COBOLs implement both CLOSE NO REWIND and OPEN INPUT REVERSED for single reel files. The read reversed was a function used by tape sorts where the work files were on tape. AIX COBOLs accept and ignore the REVERSED and NO REWIND statements. Clark Morris > >Tony H. > >---------------------------------------------------------------------- >For IBM-MAIN subscribe / signoff / archive access instructions, >send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
