>Cylinder allocation (and therefore ROUND used with average block or average >record) allows faster input/output of sequential data sets than does track >allocation.
Unfortunately, since ECKD was introduced, this statement is inaccurate. If the I/O is within a defined extent, then track/cylinder boundaries are irrelevent. Prior to ECKD, the allocation method mattered because the C/U had to check at the boundary (track or cylinder) to see whether the I/O was allowed. So, cylinder allocation was more efficient. With defined extents, either allocation is acceptable. I simplified the above explanation. IBM had an 'orange book' that explained this in great detail, when ECKD was first introduced. I managed to obtain a copy before IBM withdrew it from public distribution, because it "told the PCM's too much". - Ted MacNEIL [email protected] ---------------------------------------------------------------------- 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
