On Mon, 3 Jul 2006 12:10:08 -0500, Tom Marchant <[EMAIL PROTECTED]> wrote: > > SPACE=(10000,1024) means you want enough space to store 1024 blocks, > assuming each block is 10000 bytes. > > SPACE=(10000,1024),AVGREC=U means you want enough space to store 1024 > records, with an average record length of 10000 bytes. > > AVGREC only applies when SMS is active. when it is, as documented > in the JCL Reference, > > "The system allocates DASD space in whole tracks. The number of > tracks required depends on how the records are blocked. The system > uses one of the following as the block length to compute the number > of tracks to allocate, in the order indicated: > > 1. The block size from the DCB parameter, if specified > 2. The system determined block size, if available > 3. A default value of 4096" > > I think the system allocates more space when you specified AVGREC=U > because it took the default block size when calculating space. It > takes more space to store the same amount of data at a 4096 byte > blocksize than it does with a 10000 byte blocksize. > Which is at least consistent with the space allocated, although I might disagree with using a default average block size of 4096 when the specified average record size is 10000. Or is it presuming the possibility of RECFM=VBS? And it's just plain wrong to use the code BLKSIZE, which is a maximum, not an average, as the average block size. For ragged files, the two may differ significantly.
All the above complexity ought to whet our appetites for FBA DASD. And I did a crude mental Monte Carlo simulation. Overhead space comes in two forms, interblock gaps and unused track balances. The interblock gap consumption is pretty approximately inversely proportional to the average block size, regardless of the variance. The track balance overhead depends on whether the access method chooses to write short blocks to fill track balances (what does QSAM do?). If the access method never writes short blocks to fill tracks snugly, the overhead is 1/2 BLKSIZE per track. If the access method writes short blocks, or the file is intrinsically ragged (like a load module), the overhead depends more strongly on the maximum block size, with a smaller influence by the average. In summary, the JCL formula is attempting to estimate space based on insufficient information. It needs to be provided at least the maximum block size _and_ the average, and to utilize both. -- gil -- StorageTek INFORMATION made POWERFUL ---------------------------------------------------------------------- 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
