dyer@arktouros.MIT.EDU (Steve Dyer) (12/26/88)
Can the RT SCSI adapter be used for disks other than the 200mb and 400mb humongous "Athens" drives? This is for AOS 4.3, by the way. Looking at the driver, I suspect it just needs proper partition tables added, but I'd like to get confirmation on this before I get the adapter and a few CDC WREN drives.
buck@siswat.UUCP (A. Lester Buck) (12/28/88)
In article <2295@spdcc.SPDCC.COM>, dyer@arktouros.MIT.EDU (Steve Dyer) writes: > Can the RT SCSI adapter be used for disks other than the > 200mb and 400mb humongous "Athens" drives? This is for > AOS 4.3, by the way. Looking at the driver, I suspect > it just needs proper partition tables added, but I'd like > to get confirmation on this before I get the adapter and > a few CDC WREN drives. I have just started work on a contract to write some SCSI drivers for the RT under AIX for round tape, square tape, and a WORM drive. Will probably try out a disk driver also. (I have previously done quite a bit of Unix SCSI driver work on another system.) I haven't gotten far enough into it yet to know about substituting drives using the IBM drivers, but all those operations that read/write the "server area" on the IBM SCSI drives make me very suspicious about foreign SCSI drives working. The server area is a part of the drive where IBM stores the disk controller microcode, allowing it to be updated in place. Has anyone had much experience with low level operations on the SCSI adapter? On reading the hardware interface specifications and the VRM driver docs, a couple of things are really bugging me. 1) The SCSI adapter has 16 slots (tags) for commands active at once. It can have one "executing" command, one "next" command, and the rest "ready" to run. BUT, when the adapter is done with one command, it ALWAYS scans the list from the beginning (tag 0), so the documentation admits you shouldn't have more than THREE commands to one device out a once, as the fourth command might wait forever if the system is busy enough (say the main disk, accessed by multiple processes). This seems completely brain-damaged to me. Am I missing something? What is wrong with a simple fair-share round-robin policy, except that one can't drop a high priority SCSI command on the bus. At least have this be a setup mode of the card. 2) For some reason, the adapter is too dumb to transfer data directly between the SCSI bus and system memory, using the 128K on the adapter as a FIFO buffer. Instead, it requires intense hacking with allocation of individual 512 byte buffers on the card and "interrupt granularity" calls to break up large transfers and keep emptying the buffers on the card (the card memory is dual-ported, at least). Why can't the card just keep it simple and use the entire adapter memory as a simple buffer? 3) Why, why, why does the IBM supplied VRM driver for the SCSI adapter have a pass-through mode to do raw SCSI operations, but then emasculate it by limiting the total DMA transfer to 6K in pass-through?? Must use the separate read and write op-codes to do unlimited transfers. Very inconvenient... And using read/write op-codes causes an automatic request sense after each command - which is a performance loss and may toss some of the sense data for devices which return a lot of sense data. 4) Why are linked commands not supported? 5) Why, if we want to abort a single thread to a single LUN, does the adapter insist on possibly reseting the entire SCSI bus??? Yeow! This is not an elegant SCSI host adapter. I am looking at substituting the Western Digital 7000-asc or Adaptec AHA-1540 AT host adapters on the RT - much cheaper and better implementations. -- A. Lester Buck ...!uhnix1!moray!siswat!buck