Ron, Just to be sure someone mentions this;
Compressed Format sequential data sets are a special case of PS-E's. >From "Macro Instructions for Data Sets': "Recommendation: For compressed format data sets, do not specify NCP (thus, allowing the system to default it to 1) or specify NCP=1. This is the optimal value for NCP for a compressed format data set since the system handles all buffering internally for these data sets. " Best Regards, Yifat -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Ron Hawkins Sent: Wednesday, January 27, 2010 10:28 PM To: [email protected] Subject: Re: why compression costs additional I/O? Peter, Yes for your example I am recommending NCP=96, which means BUFNO=96. I habitually put both NCP and BUFNO on BSAM files because I've never been sure if BSAM calculates BUFNO using the NCP value from JCL. Many years ago I tested this to death on uncached DASD and found that BUFNO/NCP of 16 was the point of diminishing return for QSAM and BSAM. While I don't think these double buffer by design like EFS I think it fit well with the chain length limit of eight blocks with BSAM and QSAM. I should revisit this as a study on FICON and Cached DASD as it is likely that the knee in the curve happens at eight buffers now as I've noticed CPU intensive utilities like IEBDG writing short chains when volumes are SIMPLEX, and full chains when TrueCopy synchronous delays are added with DUPLEX. It suggests to me that 16 is still a good number for when IO is delayed. Thirty-one would be something I would recommend for BUFND on a VSAM file with half track CISZ, but I don't think it does any harm on DSORG=PS. As far as I recall BSAM and QSAM for PS-E does not have the same SSCH data length and #CCW restrictions as PS, and media manager is probably limited to a CYL. I'd only wish I had time to research this as a "science project" right now, but at the moment I can only offer past experience with a spattering of senior moments. Ron ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
