On Sun, Mar 07, 2021 at 01:13:26PM -0500, Joseph Reichman wrote: > That’s what I’m trying I.E overlapped i/o > > BSAM
In the old days with real "count key data" (CKD) disks the fastest you could read a disk depended on the disk rotational speed and it was only possible to run one read per disk at a time. QSAM could run that fast and did chaining of channel programs to minimize I/O & CPU required. In that case BSAM wasn't any faster and without careful coding could easily be slower or have errors like losing blocks or order of blocks. Today, I suspect that the CKD disk is virtual and simulated on multiple(?) fixed block disks so perhaps it might be possible to transfer data faster. This still in my estimation likely to make QSAM a better choice than BSAM and not any slower. Remember that QSAM can assume you will continue reading sequentially (and read ahead) while BSAM may think it's likely but not sure until you issue the next READs. > In other words you don’t think bufno / ncp > And overlapped I/O will produce anything significant I'd think that QSAM is already doing that if you specify a high enough BUFNO in any of DCB, JCL DD card DCB subparm, or dynamic allocation key DALBUFNO. And if performance was important, I'd make measurements of what was happening on a fine grain basis (time I/O? and CPU) and compare with the hardware specifications. And then if they didn't match try to figure out why... ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
