In a message dated 6/22/2005 7:23:43 A.M. Central Daylight Time, [EMAIL PROTECTED] writes:
Only certain drives. The S/360 DASD and the original S/370 DASD did not have skip displacement, although they did have alternate tracks. IIRC, the 3330 was the first DASD that had skip displacements. Before leaving the factory, a new disk was subjected to rigorous testing in a very sensitive environment. If a bad spot was found on a track that was small enough to be covered by skip defects, one or more defective spots were indicated on the track by recording their offset from the track's index point in special 2-byte fields called skip displacements. The controller had logic to look for this info on the track and, if found, it would automatically skip over the defective spots thus marked. If a defective spot was too large to be covered by all possible skip defects on one track, then an alternate track might be assigned or, if the platter had too many such tracks, the entire platter would be rejected. All skip defects found and assigned in the factory were recorded in the diagnostic tracks area, accessible only to authorized programs. The max number of defects per track grew larger over the years as recording densities got higher. I think the 3330 had one, 3340s and 3350s had three, and 3375s, 3380s, and 3390s allowed up to seven skip defects per track. But all that is arcane virtual history now, as no new SLED 3390s have been manufactured by any vendor for many years. Only RAIDed FBAs are produced now. Bill Fairchild ---------------------------------------------------------------------- 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
