CLAYTON@XRT.UPENN.EDU ("Clayton, Paul D.") (09/20/87)
Information From TSO Financial - The Saga Continues... Chapter 23 - September 19, 1987 John Macallister has asked the following question regarding the mixing of RA81 and RA82 type devices on the same requestors in an HSC. We're installing an HSC50 and have a mixture of RA81/RA82 disks. To balance the load it would be necessary to mix RA81's and RA82's on the same controller. Any known problems with this given that the disks have slightly different access times? While it 'sounds' right to mix the RA81 and RA82 devices on the various requestor cards in an HSC I STRONGLY recommend AGAINST it. If this is suggested by your local field service office, suggest to them they read the internal DEC mail from DEC West Disk gurus. The HSC code SHOULD be at rev. level 3.5, or 3.51 if you have shadowed devices, and there is a SPECIFIC order to the requestors now. I talked about this order in a past article of mine and some problems (horrors) I have been having. The order is as follows. CPU CI Link Module Requestor Card(s) For Tape Devices . . Requestor Card(s) For RA81 Devices . . Requestor Card(s) For RA82 Devices . . Now there is nothing you can do about combining RA81 and RA82 devices on the same requestor card when crossing from one type to the other. If the above sequence is not followed, the result, at least in my case, was somewhere between 1 and 100 'SDI Collision' errors per day. DEC resequenced my requestors and devices and there has not been a 'SDI Collision' error for weeks. The other area that you will be finding out is that the HSC50 just isn't fast enough setting up the commands for the faster drives. The PDP11 inside the HSC is performing I/O decomposition, sequencing and disk management. We have RA81, SI83C and SI93C drives and in comparing the 'throughput' between an HSC50 and 70 the 70 will perform 10 to 20% more I/O's then the 50. It also needs to be pointed out that the actual transfer of data to/from the disk/CI is NOT done by the PDP11, but is performed by the bit-slice processors on the requestor cards themselves. In summation, with the faster drives, more I/O's can be completed in the same period, and this creates a new bottleneck, that being the HSC. If Shadow Set software is in use the bottleneck can be worse because extra comparisons are done in generating the 'sequencing' of I/O command packets to the disks so that you receive the data from the disk that has it closest at hand. It has never been said, that I know of, that VAXClusters are easy to understand. This area proves it once again. Hope the information helps. Paul D. Clayton - Manager Of Systems TSO Financial - Horsham, Pa. USA Address - CLAYTON%XRT@CIS.UPENN.EDU