From owner-aic7xxx Wed Oct 7 11:32:22 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id LAA18032 for aic7xxx-outgoing; Wed, 7 Oct 1998 11:32:22 -0700 (PDT) (envelope-from owner-aic7xxx@FreeBSD.ORG) Received: from safir.spray.fi (safir.spray.fi [195.10.150.130]) by hub.freebsd.org (8.8.8/8.8.8) with SMTP id LAA17927 for ; Wed, 7 Oct 1998 11:32:06 -0700 (PDT) (envelope-from oa@spray.fi) Received: (qmail 926 invoked from network); 7 Oct 1998 18:31:43 -0000 Received: from ws142.spray.fi (195.10.150.142) by safir.spray.fi with SMTP; 7 Oct 1998 18:31:43 -0000 To: Michael Meissner Cc: Paul Haigh , "aic7xxx@freebsd.org" Subject: Re: Synchronous/Asynchronous and Disconnect/Connect? References: <01BDF175.B29F64A0.paul@nailed.demon.co.uk> <19981007081601.A25700@tiktok.cygnus.com> Mime-Version: 1.0 (generated by tm-edit 7.106) Content-Type: text/plain; charset=US-ASCII From: Osma Ahvenlampi Date: 07 Oct 1998 21:31:50 +0300 In-Reply-To: Michael Meissner's message of "Wed, 7 Oct 1998 08:16:01 -0400" Message-ID: Lines: 47 X-Mailer: Gnus v5.6.43/Emacs 20.2 Sender: owner-aic7xxx@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org Michael Meissner writes: > Basically Async is the original SCSI-1, Sync is newer and faster ('Fast' is the > 20 Mb/s improvement, 'Ultra' is the 40 Mb/s bus speed). Disconnect is for slow > devices not to hog the scsi bus. Where an extreme example of a "slow device" is something like a tape drive rewinding. Without disconnect, nothing else could be done on the bus until the tape had rewound itself and the command had returned. With disconnect, the initiator can tell the target to let go of the bus and send a reply back once they're finished, so the initiator can use the bus with other devices meanwhile. Also, Fast/Ultra are bus clock speeds, whereas the transfer speed is a function of bus clock speed and bus width: bus speed narrow wide SCSI-1 5 MHz 5 MB/s N/A Fast 10 MHz 10 MB/s 20 MB/s Ultra 20 MHz 20 MB/s 40 MB/s Ultra2 40 MHz 40 MB/s 80 MB/s Asynchronous transfer are without a clock signal, and are always somewhere below the maximum synchronous speeds. For SCSI-1, I never saw async speeds better than 3.7MB/s or so even on the best implementations (which were very close to 5MB/s in synchronous mode) Incidentally, 10 Mb/s == 1.25 MB/s, since b == bit. SCSI has parallel data lines (for now, anyway), so its speed is never measured in bits per second, unlike the serial connections like Ethernet, Fibre or USB. > No, what you are missing is the scsi bus speed (40 Mb/s) is the maximum > possible data movement for all devices connected to the bus (ie, you might be > able to reach 40 Mb/s if you have 3-4 cheetah's all tallking at the same time). > It's the guaranteed not to exceed speed, not the average speed you will see. Well, in benchmarks ran on Beowulf data servers (I've seen the results somewhere on beowulf.org, although now I can find data only for IDE disks), it became pretty apparent that more than 3 disks per controller, and the sustained speed flattened significantly before the theoretical maximum bus capacity. 4 Cheetahs on the same bus probably won't get you 40 MB/s, while the same drives on two Ultra-Wide controllers probably would surpass 40MB/s aggregate speed in a fast machine. -- Paul's Law: You can't fall off the floor. Osma Ahvenlampi To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-aic7xxx" in the body of the message