From owner-freebsd-hackers Tue Nov 10 22:58:47 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id WAA10437 for freebsd-hackers-outgoing; Tue, 10 Nov 1998 22:58:47 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from apollo.backplane.com (apollo.backplane.com [209.157.86.2]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id WAA10431 for ; Tue, 10 Nov 1998 22:58:46 -0800 (PST) (envelope-from dillon@apollo.backplane.com) Received: (from dillon@localhost) by apollo.backplane.com (8.9.1/8.9.1) id WAA18122; Tue, 10 Nov 1998 22:58:03 -0800 (PST) (envelope-from dillon) Date: Tue, 10 Nov 1998 22:58:03 -0800 (PST) From: Matthew Dillon Message-Id: <199811110658.WAA18122@apollo.backplane.com> To: john cooper Cc: grog@lemis.com, hackers@FreeBSD.ORG, mike@smith.net.au, ticso@cicely.de, john@isi.co.jp Subject: Re: SCSI vs. DMA33.. References: <98Nov11.134648jst.21907@ns.isi.co.jp> Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG The biggest issue with IDE is that it's a serialized interface. You can only run one command at a time to one device at a time and you have to wait for it to complete before you can run another command to the same device or another device. The throughput is therefore irrelevant, since most disks can't do more then 9 MBytes/sec off the platter anyway (note: the latest bleeding edge IBM drives can apparently do 20 MBytes/sec or better off the platter). IDE also isn't too hot when it comes to bad sector remapping or multiple devices, which is why most PC's come with two separate IDE busses these days. One for the CDRom, and one for the disk, nor is IDE necessarily reliable when sharing a bus in a master-slave configuration, even if both drives are the same brand. On the otherhand, a 40 MByte/sec SCSI bus with four 9 MByte/sec drives on it can actually be doing 36 MBytes/sec on the bus. Since, typically, anything approaching that kind of bandwidth is also an indication for the need for performance, it seems silly to me to even begin to compare IDE with SCSI no matter what the DMA transfer bandwidth of the IDE device is. I suppose if you never needed to seek the drive it might matter, but the moment you start needing to seek the drive the platter bandwidth goes to pot. The best a fully saturated randomly seeking disk can do is typically less then 2 MBytes/sec using 16K reads, and it doesn't get much better with larger reads. It takes truely large reads (256 KBytes or larger) between seeks to even approach the platter's bandwidth. The size of the disk in the seeking case is irrelevant, really, because voice-coil technology has not gone through the same insane technology leaps that the disk heads have gone through. IDE is still useful when you don't need performance, and I would say that 80% of the server installations these days fall into the 'don't need the performance' category. My home machines are a mix of IDE and SCSI, but all of the rack mount FreeBSD boxes in BEST's machine room are SCSI-only. If you do need performance, you go with SCSI. -Matt : :Hi, : Just wondering if anyone has any _objective_ opinion on :the performance of say wide SCSI2 vs. DMA33 IDE drives [running :on contemporary motherboards]. The theoretical throughputs of :40MBs and 33MBs don't tell me a whole lot. I know SCSI was the :choice for performance in the past, however I'm curious what :others are seeing in actual usage these days. : :Thanks, : :-john : :To Unsubscribe: send mail to majordomo@FreeBSD.org :with "unsubscribe freebsd-hackers" in the body of the message Matthew Dillon Engineering, HiWay Technologies, Inc. & BEST Internet Communications & God knows what else. (Please include original email in any response) To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message