FreeBSD/FDisk geometry problems - SOLVED!
Keith Kelly
c0d3h4x0r at hotmail.com
Fri Jan 23 22:27:05 PST 2004
I've found a bug in FDisk which is responsible for all the problems I've had
trying to get FreeBSD installed. I also found a work-around, and I'm happy
to report I'm typing this message from Konquerer inside FreeBSD 5.1-RELEASE
right now.
Basically, the problem is that FreeBSD's FDisk and the motherboard BIOS
independently calculate a set of CHS values (Cylinders/Heads/Sectors) based
on the total sector count of the disk, but they do it in different ways and
thus end up with different values.
My hard drive is a Maxtor 5T040H4. This is a 40GB ATA/100 IDE drive.
Maxtor reports the physical geometry as 79408/16/63, which yields a total of
80043264 sectors. With a large drive like this, the catch is that 79408 is
too big to fit in the "cylinders" field in the BIOS, so to make the drive
work, you have to calculate an "equivalent" set of CHS values (decrease the
cylinders value, while keeping the total sector count the same). For
anyone who doens't know, the formula is like this: cylinders x heads x
sectors = total sector count.
My motherboard (MSI KT4 Ultra) BIOS calculates 19618/16/255 (80041440 total
sectors). FreeBSD's FDisk calculates 4982/255/63 (80035830 total sectors).
You'll notice that the total sector count is not the same, and you may
wonder why. It's because of rounding error and the fact that the
calculations were done in reverse. In theory, either set of CHS values
should work fine, but the problem is that my BIOS picks one set and FDisk
chooses another set -- and FDisk refuses to accept and use the set my BIOS
calculated.
For instance, my BIOS starts with the REAL total sector count of 80043264.
It assumes a sector count of 255, and it assumes a heads count of 16. So it
calculates cylinders as 80043264/(255x16)=19618.44706, which rounds down to
19618. My BIOS does all this calculation automatically for me because I
chose "Auto" for the drive in the BIOS.
FDisk also starts with the REAL total sector count of 80043264. But it
assumes a sector count of 63, and it assumes a heads count of 255. So it
calculates cylinders as 80043264/(63x255)=4982.462745, which rounds down to
4982.
So, the problem is that FDisk makes *different* assumptions than my BIOS
does about what the sectors and heads values should be. I ran across some
information on a BIOS manufacturer's site which claimed that for "LBA mode"
SCSI drives (more accurately known as "LBA-Assist translation mode"), that
it is safe to assume that sectors should be 63 and heads should be 255.
Given that FreeBSD's roots and developer community seems historically
SCSI-centric, I can see how these assumptions would have been picked up and
used in FDisk and considered acceptable. But these assumed values are
clearly not correct for how CHS gets calculated by many PC BIOSes for IDE
drives.
Furthermore, I believe that the reason FDisk rejects the manually entered
CHS of 19618/16/255 is because either (1) it tries to enforce those bad
assumptions about heads and sectors, or (2) it gets confused by the rounding
error. In other words, in the case of rounding error, FDisk may be taking
the manually-entered values, multiplying them together, and seeing that it
doesn't exactly match (or come close enough to, in its humble but flawed
opinion) the total sector count for the drive. The way Fdisk's geometry
validation ought to work is like this:
- Divide the total sector count of the drive by (H*S), where H and S are the
user-supplied values.
- Round the result to the nearest whole number.
- Compare that result to the user-supplied value for cylinders.
- If the result matches, accept the user's input as good.
I hope that a developer somewhere can take this information and put it to
good use. I would be very happy to test a fix if someone can implement it.
In the meantime, the workaround for anyone experiencing this problem is to
go into their BIOS and set the hard drive to "User" mode, and manually enter
the same C/H/S settings that FDisk calculated for the drive. Unfortunately,
I think this means that if you have to repartition and reformat the entire
drive, since the BIOS will now be addressing the drive using different C/H/S
settings and will be unable to read any partitions that were formatting
using different C/H/S addressing. So while there is a workaround, it is far
from an ideal user experience.
- Keith F. Kelly
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