Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!cbmvax!daveh From: daveh@cbmvax.commodore.com (Dave Haynie) Newsgroups: comp.sys.amiga.hardware Subject: Re: Hard disk transfer rates Message-ID: <14330@cbmvax.commodore.com> Date: 10 Sep 90 19:49:13 GMT References: <126198@pyramid.pyramid.com> Reply-To: daveh@cbmvax.commodore.com (Dave Haynie) Organization: Commodore, West Chester, PA Lines: 48 In article <126198@pyramid.pyramid.com> telam@pyrps5.pyramid.com (Thomas Elam) writes: >In article <1990Sep1.231510.10650@agate.berkeley.edu> c60c-1gd@e260-1c (Joon Song) writes: >>I've read several articles >>claiming that hard disk transfer rates through a SCSI port is as high as >>3.5 meg/sec. Is this the actual throughput of the hard disk system? >I was confused too, until I remembered seeing disk I/O throughput >measurements on our Pyramid mainframe computers. They were around 400 >Kilobyte per second or higher, measured by user-level software, so this >*is* the throughput of the hard disk system. This works out to be 3.2 >Megabits per second (400,000 x 8 = 3,200,000) or higher. I believe >some people thought SCSI systems have throughputs of 4 Mega-*bytes* >per second. I'm not on the SCSI committee, but I'll bet an Amiga that >the throughputs are really about 4 Mega-*bits* per second. Typical SCSI devices use the asynchronous transfer mechanism, which is limited to about 1.5 MegaBYTES per second. Synchronous SCSI, which uses a clock rather than a handshake to clock its data, runs at a peak of 4-5 MegaBYTES per second. Currently, you won't find actual disk drives that can get data off their platters much faster than the 1.5 MegaBYTES per second of the asynchronous SCSI, and most are slower. But you don't necessarily have just one device connected to the SCSI bus, so if a drive is capable of buffering up a track and sending over SCSI a 4 MegaBYTES per second, rather than going directly from the disk at 1.5-something MegaBYTE per second, you have a big win in a system with multiple SCSI devices. >I can't address myself to your other questions, but I have seen >calculations made by UNIX-guru-grade I/O software engineers that work >like your calculation. And I think the numbers are right, too. UNIX is a bad example of "high speed disk I/O", in general. Most fast UNIX systems at the Workstation or PC level manage about 400 KB/s, or less. Under the Amiga OS, it's not all that unusual to have typical 800 KB/s transfers, and with a suitable hard disk (like of the Wren drives), you can get noticably over one MB/s. That is, of course, though the filing system. Part of the problem with UNIX is that most transfers are broken up into small blocks. You might have 10 separate I/O operations to load a 10K block, even if its contiguous out on disk. The Amiga's FFS will permit loading of a block of any size in one operation. >Tom -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Get that coffee outta my face, put a Margarita in its place!