On 29 Apr 2007 08:29:05 -0700, in bit.listserv.ibm-main you wrote: >> If you ask about DITTO - well in my understanding DITTO has no idea >> about real BPI. How DITTO can measure length of the tape and number of >> bytes written, especially when compression is turned on? Can DITTO >> access number of metres of tape rewound from any counter in the drive? >> I can be wrong, but I believe DITTO provides the value from some >> table, rather than from measurement. I can be wrong here. >As for BPI, you are correct. There is no hardware query that provides >it. For our FATS/FATAR products, we depend on info from the various >vendors (various = 2) for the approximate BPI. > >For tape length, 3480s had a fixed length cartridge. 3490Es had a >double length cart, but it provided a query to tell you what kind of >tape was mounted. For more modern systems, the queries have gotten >complex but they providea lot of info about the tape. > >But for your second question, there is a way to get the compressed bytes >written and read on the tape. It works even back to 3480s with IDRC, so >we can calculate approximate tape length used, and estimate percentage >used.
The blocks are recorded at the byte density. Interblock gaps reduce the average byte density for the tape (as number of blocks increase average density decreases). This is why there was a change to z/OS to allow larger blocks. Compression merely stores the data in fewer bytes on tapes increasing effective density. ---------------------------------------------------------------------- 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
