Xref: utzoo comp.sys.ibm.pc:15485 comp.sys.misc:1409
Path: utzoo!attcan!uunet!husc6!cmcl2!nrl-cmf!ames!lll-lcc!pyramid!octopus!pete
From: pete@octopus.UUCP (Pete Holzmann)
Newsgroups: comp.sys.ibm.pc,comp.sys.misc
Subject: Re: RLL- why it is hard on drives (was Re: Disk Controller Question ?)
Summary: Tight window margins are needed. Drive won't "go bad" permanently
Message-ID: <216@octopus.UUCP>
Date: 12 May 88 10:21:39 GMT
References: <1255@kodak.UUCP> <638@mccc.UUCP>
Reply-To: pete@octopus.UUCP (Pete Holzmann)
Organization: Octopus Enterprises, Cupertino CA
Lines: 94

In article <638@mccc.UUCP> pjh@mccc.UUCP (Pete Holsberg) writes:
}In article <1255@kodak.UUCP> crassi@kodak.UUCP (charlie crassi) writes:
}...I was told that the RLL controller will work fo about a month and then     
}...will destroy the media which is not coated thick enough. Is there any
}...truth to the matter.
}
}
}Your data is living on borrowed time!  Any medium which is not certified
}for RLL will eventually fail (unless you are VERY lucky!) because of the
}demands an RLL controller makes on it.  The horror stories told on
}CompuServe were enough to convince me to stop using my Adaptec
}controller with an ST-225.  People have reported that they were not able
}to reformat their disks with their MFM controllers after abandoning
}RLL!!  This is not to castigate RLL, but an RLL controller needs a drive
}that can handle the storage density and waveform requirements.

Sorry, but this response is only half true!

True things:	- ST225 drives do not work well with RLL
		- A drive that does not work with RLL will develop 'bad tracks'
			over time. These *can* be fixed by reformatting.
		- People sometimes end up in panic mode after RLL fails. There
			are many failure modes for setting up a hard disk;
			some people may not be able to reformat their drives
			to MFM, but it is not the fault of the RLL controller!
			(Unless it had some kind of electrical defect).
		- An RLL controller needs a drive that can handle the
			*waveform* requirements.

False things: (I'm going to give the opposite, true statements):

		- Non-certified drives OFTEN work just fine, forever! Some
			certified drives are marginal, and can't handle RLL
			for that matter.
		- "Storage Density" (physical flux changes per area) is no
			higher on an RLL drive than on an MFM drive.

I guess it is time to shed some more light (hopefully!) on this subject.

MFM formatting is *actually* a form of RLL. What we call 'RLL' is just a
different encoding scheme. It increases the amount of data stored on the
disk by placing the flux changes on the platter in a more accurate pattern
than that used by MFM. The DENSITY of flux changes is not any different.
RLL simply requires more accuracy in TIMING. The term used is 'window
margin'. Essentially, there is a window of time within which the drive+
controller must decide whether or not there is a flux change on the media
surface. When using RLL, this window is smaller: the timing of the flux
changes must be more accurate. 

So: low quality disk media produces poor window margins, because of the
low density of magnetic particles, etc. Cheap drive electronics produce poor
window margins, because of loose tolerances. Poor RLL controller design can
do the same thing: some controllers contribute less to window margin error
than others.

What does this mean?

	- RLL (or any other encoding scheme for that matter) does not
		physically damage or change the disk drive in any way
	- Some lower-quality drives should NEVER be used for RLL, because
		they don't have good enough window margins. This includes
		Seagate ST225 (and probably should include the ST238; it is
		'certified' by Seagate, but often has trouble anyway).
	- Many high quality drives, especially those with plated media and
		all-around good electronics, will work without trouble!
		When you buy 'certification' from these manufacturers, you
		are NOT buying a different drive. You are simply paying
		extra for a guarantee of stricter tolerances. Depending
		on the drive involved, this guarantee may be essential
		[ST225/238- only a few drives pass the test] or it may
		be completely unnecessary [Maxtor drives work GREAT with
		RLL, even though not 'certified'] or something in between.
	- There are tests that can be performed on any drive/controller
		combination to see if RLL will work well. It simply involves
		measuring the window margin. The window margin can't be fully
		exercised without specialized hardware: You can't change
		the timing of your PC's disk interface beyond its worst
		case.
	- On the other hand, if you perform an intense read/write/format test
		on your drive, using worst case data patterns, you can develop
		a good level of confidence in your system. This is what the
		SpinRite program does: It beats up on the inner track of a
		drive [inner track has highest flux-change density] with a
		bunch of worst case data patterns. If this passes, there is
		little or no cause for alarm.

Hope this clears things up a little!

Pete
-- 
  OOO   __| ___      Peter Holzmann, Octopus Enterprises
 OOOOOOO___/ _______ USPS: 19611 La Mar Court, Cupertino, CA 95014
  OOOOO \___/        UUCP: {hpda,pyramid}!octopus!pete
___| \_____          Phone: 408/996-7746