I'm not sure why you think these are competing efficiencies. A large physical block reduces waste on the track by requiring fewer Inter-Block Gaps (IBG) and it reduces the number of I/O's required to read the data.
The problem is that the maximum blocksize was limited by software to 32K. Since this isn't an even multiple of the track capacity on a 3390, the best blocksize would be a half track. SDB provided the means of letting the system calculate this without requiring the programmer to do the calculation. However, such an "optimum" blocksize is only relevant when processing sequential files. VSAM uses the CISIZE, so blocksize is irrelevant. For most PDS's there will be more short blocks because members are rarely large enough to use a half-track block. In load libraries, most of the records are irregular sizes, significantly smaller than the blocksize, so again, it doesn't do anything. Adam ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
