Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!mcsun!unido!mpirbn!p554mve
From: p554mve@mpirbn.mpifr-bonn.mpg.de (Michael van Elst)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Hard disk transfer rates
Message-ID: <1246@mpirbn.mpifr-bonn.mpg.de>
Date: 26 Sep 90 00:38:46 GMT
References: <1938@lpami.wimsey.bc.ca> <2253@impch.imp.com> <2434@bambam.UUCP> <24541@uflorida.cis.ufl.EDU> <1245@mpirbn.mpifr-bonn.mpg.de> <24562@uflorida.cis.ufl.EDU>
Reply-To: p554mve@mpirbn.UUCP (Michael van Elst)
Organization: Max-Planck-Institut fuer Radioastronomie, Bonn
Lines: 41

In article <24562@uflorida.cis.ufl.EDU> jma@beach.cis.ufl.edu (John 'Vlad' Adams) writes:
>Sure, the Quantum has a 64k cache, but I certainly hope that a Quantum
>is capable of better than 450k writes when all I've heard is how
>a Quantum is one of the best low-end drives.  I also would hope that
>a DMA controller like the 2091 is faster than a non-DMA GVP card
>using a bloody slow Segate drive.  I'm not satisfied with the
>64k cache being the only saving grace of the drive.  I guess I
>could drop my Quantum onto his GVP card sometime, although
>it would be one heck of a hassle with my parition setup.
Well, harddisk timings are a little difficult. There's the 'raw'
data transfer rate from the drive (should be about 930K/sec for the
Quantum) which is the theoretical maximum that can be achieved.
The problem is that you may miss the next sector to read if anything
interrupts the transmission. This can be a step to the next cylinder
or the delay when another SCSI command is sent. What happens ? Without
a cache you'll really miss the next sector and wait a complete revolution
of the disk (thus the sustained data rate will be half of the maximum).

What can be done ? To minimize the delay by the step you can introduce
a track skew so that the next sector to read will just pass under
the read/write head when the heads have settled onto the new track.
Another method is to start reading at an arbitrary sector (which can't
be transmitted yet) into a cache and start transmission when the
right sector has been read. When the track has been read completely the
drive can pump all pre-read sectors with the maximum data rate of the
SCSI Bus. Even this takes some time which can be used to read ahead
some sectors (maybe of another track) so that reading the drive will
be more or less continously and approaches the raw data rate.

Unfortunately, this is a little tricky to do when writing.

>Or is GVP compatible with the 2091's RDB?
Since RDB is a standard :-) any controller that supports RDB should
allow to access any drive that has the RDB written on it.


-- 
Michael van Elst
UUCP:     universe!local-cluster!milky-way!sol!earth!uunet!unido!mpirbn!p554mve
Internet: p554mve@mpirbn.mpifr-bonn.mpg.de
                                "A potential Snark may lurk in every tree."