So what gets sent down the channel? If I put BLKSIZE=1024 on my JCL for the output DD statement of the DFDSS dump job, will DFDSS dutifully send 1K blocks down the channel to the tape controller/drive and the hardware builds big blocks and writes the large block to the tape, or does DFDSS "know" it's sending data to the buffered drive so it builds large blocks to send down the channel to the drive in anticipation of the drive wanting large blocks?
Rex -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf Of Staller, Allan Sent: Monday, May 12, 2014 2:13 PM To: IBM-MAIN@LISTSERV.UA.EDU Subject: Re: Performance for DFDSS with ORACLE Tape Drives VSM5 ( was Change tape block size) Agreed. Low compression ratios will produce more "blocks" to actually be written to the tape, elongating total application time. <snip> W dniu 2014-05-12 21:02, Staller, Allan pisze: > It is my recollection that, pretty much since the advent of buffered > tape drives (e.g. 3590, 9840,... ) the actual blocksize written to tape is > independent of what is specified in the JCL. > > In other words, it does not matter what you code in JCL, the actual data > block written to physical tape will be a hardware determined value. Well, I performed some tests on various real and virtual drives. Although the JCL DD BLKSIZE is not actual physical blocksize on the tape, but the BLKSIZE still does matter. What is also important, very important is compression ratio, but in real life it depends on your data, so there is no much to tune here. </snip> ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN The information contained in this message is confidential, protected from disclosure and may be legally privileged. If the reader of this message is not the intended recipient or an employee or agent responsible for delivering this message to the intended recipient, you are hereby notified that any disclosure, distribution, copying, or any action taken or action omitted in reliance on it, is strictly prohibited and may be unlawful. If you have received this communication in error, please notify us immediately by replying to this message and destroy the material in its entirety, whether in electronic or hard copy format. Thank you. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
