Where's Mark Thomen when we need him. This is really a little out of my area and when it is I tend not to respond, but I'll have a go anyway...
On Thu, 26 Jul 2007 11:06:36 -0500, McKown, John <[EMAIL PROTECTED]> wrote: >I'm curious about this one. A person here has stated that when a VSAM >KSDS file is in multiple extents, it performs more poorly than it would >in a single contigueous extent. He said something to the effect that >when a portion of the index is referenced which is in "another extent" >that the entire in-memory buffer for the file is flushed. I have no idea >where he got this information. Nonsense. I have seen some benchmarks with extents vs. no extents and there is perhaps some small performance saving but I think the bottom line was it was very minimal. And I know one of the line items for z/OS 1.5 (I think that was when it came in) - VSAM extent consolidation - mentioned improved performance, but I think the benefit was more towards decreasing the chance you would run into the 123 extent volume limit than performance improvement. > >Also, he wants to use SMS striping for VSAM files which are mainly >accessed sequentially because it would significantly decrease the I/O >time to read. We are using multiple FICON connections to a single 2105 >ESS (Shark). We do not have the volumes in any storage group spread >across LCUs. My contention is that it may well decrease I/O, but it >would require that we likewise stripe the volumes across the 2105's LCUs >as well. We don't do this at present. But striping will still allow parallel I/O to multiple devices. Consider this analogy. You have a 4 volume multi-volume data set all on that same LCU. Can you dump them quicker by running 4 full volume dumps single threaded, or by running 4 at a time - one per volume. BTW, have you looked at the VSAM Demystified Redbook? Mark -- Mark Zelden Sr. Software and Systems Architect - z/OS Team Lead Zurich North America / Farmers Insurance Group - ZFUS G-ITO mailto:[EMAIL PROTECTED] z/OS and OS390 expert at http://searchDataCenter.com/ateExperts/ Systems Programming expert at http://expertanswercenter.techtarget.com/ Mark's MVS Utilities: http://home.flash.net/~mzelden/mvsutil.html ---------------------------------------------------------------------- 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
