John, The simple view is that with DASD, the bigger the block as a multiple of the track size, the more data you can store on a track.
It almost like an IBG on the older tapes. Best allocation or space calc is to use 1/2 track if possible, for QSAM, and PDSs. For PDSEs using 32760 is fine because I believe they like linear VSAM files with 4K blocks. Remember for DASD, the maximum block size ( physical record ) is 32760. But this means on a 3390 you waste about 24K per track. Where as, using something near 27998 or less, you will get 2 blocks per track with about 1% overhead. It looks like PDSEs have about 13% overhead per track assuming a 4K (PAGE) track record size ... BUT no compression needs.. For the most part, let the OS handle the allocation by using BLKSIZE=0 where you can. It will calc the optimized track record size ( blksize ) for the specific device, including tapes. Do not expect 100% track utilization with MVS/VSE/VM ... ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
