Path: utzoo!mnetor!motto!ecijmm!eci386!ecicrl!clewis From: clewis@ecicrl.UUCP Newsgroups: can.usrgroup Subject: Re: Hard Drives: SCSI or ESDI Keywords: SCSI, ESDI, disk, interface Message-ID: <704@ecicrl.UUCP> Date: 14 Oct 89 07:53:18 GMT References: <891011142510.20869@tmsoft.uucp> <700@ecicrl.UUCP> <1989Oct13.194516.3141@eci386.uucp> Reply-To: clewis@ecicrl.UUCP (Chris Lewis) Organization: Elegant Communications Inc., Ferret Division, Toronto, Canada Lines: 83 In article <1989Oct13.194516.3141@eci386.uucp> woods@eci386.UUCP (Greg A. Woods) writes: >In article <700@ecicrl.UUCP> clewis@ecicrl.UUCP (Chris Lewis) writes: >> In article <891011142510.20869@tmsoft.uucp> Tak Ariga writes: >> SCSI has a few advantages over ESDI. For example, SCSI is a peripheral >> bus, but ESDI is a *disk* bus. Hence, you can't hang your tape drive, >> or zuper-whizzbang video interface on a ESDI interface, but you can on >> a SCSI. Then again, bus contention usually makes the performance pretty >> lousy. >I'm not sure what the actual limits of the ESDI interface are, but I >know the WD1007 controller will only support 2 drives. This give SCSI >the advantage when many drives are required. ESDI physically looks like ST506 - same ribbon cables. The SCSI-host ESDI disk DPT controller supports 4 ESDI drives. Don't know if the AT bus one was only two drives or not. I've never seen the ESDI interface spec either, so I couldn't tell you the number of LUN's you can theoretically connect. >As for performance, it can be shown that using multiple drives in a >multi-user system can be a big win, when you have a smart OS (i.e. disk Which is why I said two drives (or more) for better performance than a single drive. >driver) and an interface such as SCSI 2 which will support multiple >asyncronous commands. ESDI obviously does because each of the drives has it's own cables. You also have to remember that these commands (in SCSI) aren't fully asynch... >As Chris stated, current drives don't tax >either interface in the bandwidth department, though a full complement >of 8 fast devices on a SCSI buss might be pushing the limit. Um, well, I didn't mean to imply that. 2 SCSI drives running at a hundred or two K bytes per second each will probably come close to saturating even SCSI 2. Queueing theory and all that. Which is why the preferred Spectrix configuration was a host adapter and controller *per* drive (well, the earlier controllers didn't support disconnect, but the later ones did allow overlapping seeks). >> Do NOT under any circumstances use a dumb SCSI or ESDI board that >> doesn't have it's own DMA facilities. AT DMA is ~300Kb/sec. - eg, >> *very slow*. Further, don't use controllers that do programmed I/O. > >My 3B2 SCSI host adapter has a fast 80186 onboard! Good for you. Too bad your disks don't work ;-( >The main CPU never >waits, unless that's all it has to do. Modern 3B2 systems often use >SCSI host adapters and "bridge" controllers such that you can hang 8 >bridge controllers, each with 4 large ST506 or ESDI drives, from one >host adapter. The 3B2 architecture allows up to 8 host adapters. >That's a lot of disk! >All in all, I find the SCSI architecture much more elegant.... SCSI is very elegant for relatively inexpensive solutions. But not blazingly fast.... I've worked on SCSI stuff for a long time. And had, until about a year ago said "heck, the only difference between ESDI and SCSI is that the transfer rate's faster. Why bother?" None of the machines I was working on cared. And ESDI driver's are pretty complicated. Funny thing, *nobody* we talked to was interested in SCSI. When I was working on the DPT drivers, I had a lot of contact with the DPT engineering staff (trying to find a host adapter that worked...). The DPT's I was working with were SCSI interface to the host, and ESDI to the disks. I asked him whether they'd ever support SCSI drives. The response was "are you nuts?". I know, get both worlds: buy the DPT SCSI-host version of the board, and use SCSI drivers for your ESDI disks.... (which is the same as the configuration I worked on and the 3b2's). -- Chris Lewis, Elegant Communications Inc. UUCP: {uunet!mnetor, utcsri!utzoo, uunet!attcan!lsuc, yunexus}!ecicrl!clewis Moderator of the Ferret Mailing List (ferret-request@eci386) Phone: (416)-294-9253