Date: Mon, 18 Oct 1999 20:28:32 +0100 From: Mark Ovens <mark@ukug.uk.freebsd.org> To: "Ronald F. Guilmette" <rfg@monkeys.com> Cc: freebsd-questions@freebsd.org Subject: Re: Some serious gripes about `fdisk' and also `booteasy'. Message-ID: <19991018202832.C322@marder-1> In-Reply-To: <24065.940184874@monkeys.com> References: <19991017094727.B319@marder-1> <24065.940184874@monkeys.com>
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On Sun, Oct 17, 1999 at 11:27:54AM -0700, Ronald F. Guilmette wrote:
>
> Just for your future reference, the "truth" about this is that yes, the
> backwards (harware) compatability constraints associated with PeeCee
> hardware... in which they all _still_ try to remain somewhat compatible
> with the very earliest IBM PeeCees that supported disk drives... DOES
> indeed still make us all play funny games when trying to address
> specific disk sectors, regardless of whether you have an (E)IDE drive
> or a SCSI drive.
>
> The attempts to be backward compatible mean that in the address that gets
> sent to the disk drive (in the form cylinder/head/sector) has to get munged
> around in weird ways, in particular for any disk that has more than 1 gigabyte
> capacity. Again, this bit of screwyness affects _both_ (E)IDE drives and
> also SCSI drives.
>
> But in the case of (E)IDE drives, the software driver can (I think) always
> correctly figure out the proper set of rules it has to use when forming the
> cylinder/head/sector disk address. But in the case of SCSI drives, it can't
> always do that.
>
Thanks for the info. This seemed to be in conflict with *my*
understanding of SCSI so I went searching to find out where I got
my ideas from.
The CFBSD (2nd edition) says:
"SCSI devices are a different matter.....they are always
addressed as a sequential list of sectors...."
and the "PC Hardware compatibility : Storage Devices : What is
SCSI?" section of the handbook says:
"SCSI devices are intelligent. They have to be to adhere
to the SCSI standard (which is over 2 inches thick BTW).
So, for a hard disk drive for instance you do not specify
a head/cylinder/sector to address a particular block, but
simply the number of the block you want."
However, both then go on to mention about the grief that can be
caused due to CHS translation when fdisk'ing and booting.
Seems that I got confused between addressing the disk once the
system is up and running, and getting it up and running in the
first place.
> Currently, unless you have some extra hints (e.g. a DOS partition) already
> lying around that the FreeBSD `fdisk' program can use to get a clue as to
> the current ``disk geometry'' (i.e. the rules for forming a cyl/head/sector
> address from a raw sector address) the `fdisk' program is likely to guess
> wrong about your disk geometry, at least if you are using a SCSI disk. In
> fact, now that a majority of disk drives have capacities in excess of 1 GB,
> the `fdisk' program will *mostly* guess wrong. And when it does, you *must*
> manually override its guess or else you will end up with a partition that
> you cannot actually use for anything. (And unless I'm mistaken, you _may_
> even end up improperly overwriting parts of _other_ existing partitions on
> that same SCSI disk... which would be really Bad News if you had other
> partitions on there that contained stuff that you wanted to keep.)
>
Guess I've been lucky (so far :) ). My first SCSI disk (4.3GB IBM)
fdisk'd just fine, but it does have a DOG slice on it. When I added
a second disk (4.5GB IBM) that also fdisk'd OK first time and this
one just has a single FreeBSD slice on it (although it isn't
'dangerously dedicated' as I didn't want smart-arse Windows programs
trying to "fix" a "corrupt" disk).
I've also taken a look at the BIOS setup on my adaptor (Diamond
Fireport 40 - NCR/Symbios 53c875j chipset) and there are no settings
related to translation at all. Maybe newer SCSI h/w is better at
dealing with these things automagically?
Thanks again for the info.
--
STATE-OF-THE-ART: Any computer you can't afford.
OBSOLETE: Any computer you own.
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