A few questions

Article: The Hitchhiker's Guide to the Mainframe - The Basics
By: rwessel (robertwessel.delete@this.yahoo.com), May 17, 2013 2:46 pm
Room: Moderated Discussions
Wouter Tinus (wouter.tinus.delete@this.gmail.com) on May 17, 2013 9:12 am wrote:
> Kuchibs (kuchibs.delete@this.gmail.com) on May 14, 2013 10:33 pm wrote:
> > Good basics guide. Thank you.
>
> Yes, interesting read. I have some questions though.
>
> 1) Is CPU redundancy limited to the MCM, or does it work across books as well? If the latter is
> true, it seems to me that there's no software reason why the MCM could not be hot-swapped. So
> either they don't need this feature (surely the MTBF is extremely high) or there are hardware
> reasons not to support it. Could it be too difficult to make such a swap electrically safe?


With some limitations, books can be hot-swapped. OS support is required as the number of visible cores to the OS changes during the process, as well as the amount of memory in the system. There are also some I/O connectivity issues to consider on systems with odd numbers of books. My understanding is that one or three book systems get temporarily upgraded to two or four books systems (on a three book system, only if it's the third book being swapped) during a hot swap.

Individual MCMs cannot be hot swapped, as they're not (field) separable from the book they're in.

The degree to which that's possible has varied over the years, but the current systems are generally more flexible than the older ones.


> 2) Are the I/O processors included as specialized 'cores' on the same
> die as the CPU, as chips on the MCM, or as separate devices entirely?


SAPs are ordinary cores. Some of the available cores on the system are simply assigned to that function. For example, on the current EC12, a maximum configuration has 120 cores, of which 16 (on a maximum sized system) would be SAPs (this can be increased if necessary), and up to 101 can be user visible cores (plus two spare cores, and a reserved core gets you to 120).

The SAPs run their own millicode, and may well not implement the full Z ISA when running as a SAP (as some of that requires the appropriate millicode), but SAPs are physically identical (and interchangeable) with "normal" CPs, ICFs, IFLs, zIIPs and zAAPs.

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TopicPosted ByDate
MFKuchibs2013/05/14 10:33 PM
  MainframesDavid Kanter2013/05/17 08:14 AM
  A few questionsWouter Tinus2013/05/17 09:12 AM
    A few questionsrwessel2013/05/17 02:46 PM
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