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From: scottc@bnlux0.bnl.gov (david scott coburn)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: MFM vs RLL vs IDE vs ESDI vs SCSI
Keywords: data transfer rates comparisons
Message-ID: <2132@bnlux0.bnl.gov>
Date: 19 Sep 90 17:20:48 GMT
Organization: Brookhaven National Lab
Lines: 39


There seems to be a constant barrage of numbers in the trade press and
advertising concerning data transfer rates for various types of hard
disk interfaces (MFM, RLL, ARLL, IDE, ESDI, SCSI, etc).  Can anyone
shed some light on this subject?

It seems to me that the limiting rate (assuming a fast enough processor)
would be the data rate coming off of the head-disk assembly.  For example,
MFM seems to be limited to 5 Mbits/sec, versus RLL which is 7.5 Mbits/sec.
This makes sense, since RLL has 1.5 times the linear bit density on the
disk.  I don't know how ESDI records on the disk.  On the other hand,
I have seen some SCSI interfaces (the controllers, not the disks) that
claim 16 or 32 Mbits/sec data rates.  Wouldn't the SCSI transfer rate
ultimately be limited by the recording method?

In other words, why should the data transfer rates for a RLL disk with a
host-resident RLL controller differ from a RLL disk with an embedded
SCSI controller?  Isn't this just a case of having the embedded controller
AND the host processor wait for the data to come off the disk, rather than
just the host processor?

I realize that there are a lot of factors that go into determining data
_through-put_ rates in a computer system (the io bus transfer rates, on-
board cache (on both controller cards and the embedded 'smart' drives),
efficiency of the software drivers, the mix of requests to the drive
system, etc).

If someone is aware of an article or such concerning comparative performance
of the many different types of drive interfaces I would be interested in
hearing from you.

Is there also a reasonably technical source of information on the inner
workings of the different types?  I know very little about ESDI and IDE (AT)
drives and controllers.

Thanks,

scott coburn                                brookhaven national laboratory
scottc@max.bnl.gov [130.199.128.6]                          upton, ny, usa