Geometry and Mirror

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To: FreeBSD Questions <freebsd-questions@freebsd.org>
Date: Tue, 21 Dec 2004 10:43:51 +0100

I just do not get this. I have read the manual and Im still in the fog

I used to install FreeBSD and when it came to fdisk, I have ignored
the following warning:

"WARNING: A gemotry of 310101/16/63 for ad0 is incorrect, using
a more likely geometry. If this geometry is incorrect or you are
unsure as to wheteher or not it is correct please consult the
Hardware Guide int the doc submenu or use the (G)eom command to
change it now"

It was no problem at all, since when I have started to use raid
mirror with g(eom)mirror. The problem came when after reboot
the system recognized different C/H/S values for the identical disks.
According to dmesg it was either
310098/16/63, or sometimes:
310101/16/63

The BIOS setting was autodetect for both drives. And gmirror complained,
that one drive is smaller then the other, so the mirror has been
destroyed ...

I thought I should dig this up, since its realy uneasy situation.
The fdisk warning advises to read the Hardware Guide in the doc
submenu, though this section doesn't have a word about geometry.

The Handbook Allocating Disk Space chapter says:

"FreeBSD does not use the BIOS, and does not know the ``logical BIOS
drive mapping''. This can lead to very perplexing situations, especially
when drives are physically identical in geometry, and have also been
made as data clones of one another.

When using FreeBSD, always restore the BIOS to natural drive numbering
before installing FreeBSD, and then leave it that way. If you need to
switch drives around, then do so, but do it the hard way, and open the
case and move the jumpers and cables."

Peplexing situations ... and realy

Sysinstal/fdisk says "Remember: you need to enter whatever your BIOS
thinks the geometry is!"

Ok, bios can think 3 things:

65535/16/255 CHS
4095/240/255 Large
16643/255/63 LBA

If I say CHS or Large values at the geom prompt, I receive the same
message: "WARNING: A gemotry of xxx/xxx/xxx is incorrect". Only the
LBA values are accepted, though 16643/255/63 results 130551 Mb, which
is much lower than 152625 Mb with the propused 19457/255/63 values by
fdisk. I thought I accept the values 19457/255/63, and it stores this
somewhere, though on next sysinstall it still complains about this.

So what mode (CHS,LBA,Large) should I set in the BIOS (autodetect seems
to be riksy ...) ? What geometry should I use in sysinstall/fdisk ?
Will this geometry warning always come ? Will dmesg output ever display
a correct value instead of 310098/16/63 ?

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