In <[EMAIL PROTECTED]>, on 07/28/2005 at 09:59 AM, Bill Fairchild <[EMAIL PROTECTED]> said:
>When PAV is involved, other considerations complicate matters. Any >number of I/Os can theoretically be happening simultaneously within >the same data set from any number of exposures as long as all those >I/Os are read only. But if only one of them wants to do a write, >then serialization is necessary within the control unit. In a message dated 8/1/2005 5:24:31 P.M. Central Daylight Time, [EMAIL PROTECTED] writes: >How did IBM handle that on the 2305 and the paging 3880 boxen? I don't know. That is an interesting question. Certainly the same need to serialize in order to protect data in the case of one writer and seven readers exists. Perhaps this situation didn't apply on the 3880-2305 because the only software component that ever used the multiple exposures on a 2305 was ASM (I think), and ASM could be trusted to know what it was doing with its data. The 2305 is a general-purpose PAV box, so all user I/O can go through multiple exposures (now called PAV) on it as well as ASM. The protective serialization in the 2305 is provided by the controller microcode, and applies to all I/O, including ASM, so the ASM developers are forced to make adjustments to ASM in order to accommodate what the microcode will do with ASM's highly specialized and optimized chains; i.e., unless ASM does something unusual the microcode will thwart their optimization schemes. But on the 2305 there was no need for serialization at the control unit level, since the only user was ASM and it correctly serialized through its page-handling logic. Just my guess. I would also guess that the 3880 caching controller models 11 and 21, which were intended for paging I/O only, had the same situation [1], and I think multiple exposures were available for those devices as well. Bill Fairchild [1] I.e., a potential problem that would occur if general I/O were allowed onto the device, but no problem if only ASM I/O were allowed onto it, so thus there was no controller microcode needed to handle the problem. ---------------------------------------------------------------------- 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
