Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!apple!olivea!tymix!tardis!jms From: jms@tardis.Tymnet.COM (Joe Smith) Newsgroups: comp.sys.amiga.hardware Subject: Re: Hard disk transfer rates Summary: Sustained transfer rate is not the same as peak transfer rate. Message-ID: <1233@tardis.Tymnet.COM> Date: 20 Sep 90 03:01:04 GMT References: <1990Sep1.231510.10650@agate.berkeley.edu> Reply-To: jms@tardis.Tymnet.COM (Joe Smith) Organization: BT Tymnet, San Jose, CA Lines: 33 In article <1990Sep1.231510.10650@agate.berkeley.edu> c60c-1gd@e260-1c (Joon Song) writes: >I'm sorry if this has been discussed before. 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? A lot of the numbers I've seen bandied about do not have any qualifiers; they don't say that they are peak transfer rates instead of sustained rates. >Suppose a hard disk had 34 sectors/track. A hard disk spins 60 rev/sec. >So the fastest disk transfer rate should be: > 34 sectors/track * 512 bytes/sector * 60 rev/sec = approx. 1 meg/sec. Imagine a disk controller that is mounted on the disk drive and it has a buffer big enough for a full track. Such a device could send 34*512 = 17408 bytes every 1/60th of a second (ignoring the time it takes to switch heads and assuming it has enough heads per cylinder to be worthwile). If this imbedded disk controller waited until it had a full buffer and then blasted it into the Amiga as fast as the Amiga could handle it, the manufacturer could brag about having a 3.5 megabytes per second peak transfer rate. During the 1/60th second, the average transfer speed would be: 0 bytes/sec for 11.70 milliseconds + 3.5 Mbytes/sec for 4.97 milliseconds which is only 1.04 megabytes per second. Including the track-to-track access times, the sustained transfer rate would be even lower. Summary: Be sure to specify "peak" vs "sustained" transfer rates when discussing high-speed disk performance. -- Joe Smith (408)922-6220 | SMTP: jms@tardis.tymnet.com or jms@gemini.tymnet.com BT Tymnet Tech Services | UUCP: ...!{ames,pyramid}!oliveb!tymix!tardis!jms PO Box 49019, MS-C41 | BIX: smithjoe | 12 PDP-10s still running! "POPJ P," San Jose, CA 95161-9019 | humorous dislaimer: "My Amiga speaks for me."