Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!pepper!cmcmanis
From: cmcmanis%pepper@Sun.COM (Chuck McManis)
Newsgroups: comp.sys.amiga
Subject: Re: FFS speed.  Bytes per second or Bits per second?
Message-ID: <43597@sun.uucp>
Date: 29 Feb 88 23:43:12 GMT
References: <8802251923.AA23029@cory.Berkeley.EDU> <3385@cbmvax.UUCP>
Sender: news@sun.uucp
Reply-To: cmcmanis@sun.UUCP (Chuck McManis)
Organization: Sun Microsystems, Mountain View
Lines: 34

In article <8802251923.AA23029@cory.Berkeley.EDU> (Matt Dillon) writes:
>	Considering the transfer rate for a hard disk, I don't think
> FFS can go at 600K Bytes/sec.   600Kbps-1Mbps ==  75KBps-125KBps.

In article <3385@cbmvax.UUCP> (Steve Beats) writes:
> FFS CAN GO 600K BYTES PER SECOND ON READS FROM A CONNER 100MB SCSI DRIVE!!!!!

What neither Matt nor Steve realize is that we've substituted an
instant coffee for their regular brew. Seriously, what we have is some
assumptions on the part of the claimants that disagree :-). Basically,
if you have a Hard disk and it has an ST-506 interface to the physical
drive mechanism you get around 650K Bytes per second out of the read
head tops. This is due to the fact that the ST-506 interface spec puts
the Data Clock at 5 Mhz, and 5000000/8 = 625K Bytes. In reality, the
better way to compute the transfer speed is to take the number of
sectors on a track (16) multiply that by the size (512 bytes) and
multiply that by the speed of rotation in seconds (3600 RPM = 60 RPS)
to get 512*16*60 = 491,520 Bytes/Second MAX.  But all these wonderful
formulas go out the window when the drive has an imbedded SCSI
controller because the physical drive interface may no longer be
ST-506. And in fact it usually isn't since ST-506 is out dated anyway.
Steve wasn't using an ST-506 based drive, he was using the Connors
Peripherals drive, which has a SCSI interface on it, and the actual
drive interface is something like RLL or GCR and generates bits of data
at substantially higher rates. And in truth the limiting factor becomes
the SCSI interface, which in its Asynchronous mode is limited to 1.25
Megabytes per second. So they are both right, FFS can't do 600K bytes
per second on an ST-506 drive and FFS can do 600K bytes per second on 
some SCSI drives.


--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@sun.com
These opinions are my own and no one elses, but you knew that didn't you.