I don't really know anything about this but it sounds to me like one of @Gil's cases of making a good modification in the wrong place. Why not do this is such a way as to be transparent to old QSAM programs -- no need for a DCBE? Have an operand in the JCL or in SYS1.PARMLIB or the PPT to make it behave as described below if necessary, but by default honor the JCL blocksize and hide the > 32K from the application. Tell the application the blocksize is 32K -- 15 bit integers being what they are -- but go ahead under the covers with a 260K blocksize. No QSAM program should be doing its own deblocking, and if it is, well, put an exception in the JCL or the PPT or wherever.
Or at the very least make the below behavior the default but have an option to honor the 260K even in the absence of a DCBE. Tell the customers their gun, their bullet, their feet. Charles -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Jesse 1 Robinson Sent: Thursday, February 16, 2017 8:44 AM To: [email protected] Subject: Re: 31 vs 24 QSAM I mentioned having modified a QSAM program to write 'large blocks' by replacing DCB with DCBE. My goal was to test the effect of very large blocks in our new tape subsystem, which we had learned was highly biased in favor of large blocks. This had nothing to do with AMODE, which was all 31. The program certainly ran faster with large blocks such as 260K. I could not distinguish improvement at the IOS level (lower I/O count) vs. improvement at the tape level. Most likely a combination. My problem with the new tape was that the vendor seemed to assume that a customer could just tweak JCL to create giant blocks. In fact many of our largest tape files are created by utilities--IBM or otherwise--that are not written for large blocks. In practice you can code as large a block size as you wish, but if the program contains only DCBs, any size greater than 32K is simply ignored without error. While the change to DCBE was very simple, it has to be accomplished by the program owner. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
