From: drew@kinglear.cs.colorado.edu (Drew Eckhardt)
Subject: Re: IDE, RLL, SCSI, EDSI, MFM, SCSI-2 ?
Organization: University of Colorado at Boulder
Date: Sun, 8 Nov 1992 11:37:59 GMT

In article <2227@sdrc.COM> scjones@thor.sdrc.com (Larry Jones) writes:
>In article <92308.202512U35334@uicvm.uic.edu>, U35334@uicvm.uic.edu (Juan A. Varela F.) writes:
>> I have been reading in this group a lot about the different types of hard
>> disks.  What is the difference among all these?  Which one is technologically
>> superior?  Why?  How are they compatible with each other?  I basically
>> understand nothing about hard drives and any information would be greatly
>> appreciated.
>
>
>Advantages and Disadvantages
>----------------------------
>Recording technologies are easily summed up -- higher densities give
>you higher capacity and speed and somewhat lower reliability because of
>the tighter tolerances required.  As long as the drive is designed for
>use with the recording technology, you shouldn't have any problems. 

Today's high density disks are much more reliable than yesterdays
low-density disks.

>Drives with integrated controllers increase reliability since the
>low-level signals from the disk don't have nearly as far to go.  Thus,
>a high density drive with an integral controller should be as reliable
>as a lower density drive with a separate controller.
>
>Drives with integral controllers are more expensive than drives without
>(for obvious reasons), which can be important if you're buying more than
>one.  ESDI is a nice compromise here since some of the controller logic
>is shared.  SCSI is nice if you want to support lots of devices (SCSI
>allows up to 7 devices on the bus, the other interfaces support only two
>disks) or a number of different devices.

Drives without integral controllers limit the number of sectors per 
track to whatever the standard says - ie 17 or 26 sectors per track with
MFM / RLL.  Double the capacity of a drive conforming to one of these
standards means doubleing the platers (expensive) or accuracy of the
headpositioning mechanism (expensive).  Double the density, using 
tighter timing, and you've douled capacity without a significant 
increase in cost (to the manufacturer).  So, since the higher-density
drive uses fewer expensive mechanical components to achieve the
same capacity, it can be cheaper.

>Performance is very difficult to generalize.  It depends as much on
>what you are doing and what kind of software is driving the hardware
>as it does on the actual hardware.  As a very rough rule of thumb,
>ST506 interface drives are slowest, IDE, SCSI, and ESDI fastest.  For
>multiple disks on a multitasking operating system, nothing beats a
>top-of-the-line bus mastering SCSI controller.

The command overhead on some SCSI devices can be significant.  
If your operating system isn't caching, and doing reads/writes 
with a reasonable number of sectors, you'll suffer.  IDE lacks 
this command overhead, and all things equal, is probably
better in a single disk system in terms of both performance and
price.

However, with IDE / MFM / RLL / ESDI drives, you can only talk to 
one at a time.  In a multidisk system, when one disk is seeking, 
you can't touch the other disk.  SCSI devices will disconnect from the 
bus when they're seeking, etc, so that other devices can use it.  This
will let you keep a number of disks busy at the same time... 

-- 
Microsoft is responsible for propogating the evils it calls DOS and Windows, 
IBM for AIX (appropriately called Aches by those having to administer it), but 
marketing's sins don't come close to those of legal departments.
Boycott AT&T for their absurd anti-BSDI lawsuit.
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