*BSD News Article 99436


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From: wpaul@ctr.columbia.edu (Bill Paul)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: Big EIDE disk, old IDE controller
Date: 11 Jul 1997 14:27:22 GMT
Organization: Columbia University Center for Telecommunications Research
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Daring to challenge the will of the almighty Leviam00se, Timothy J. Lee
(nobody@not.for.email) had the courage to say:

: Are there any potential problems with using a big ( > 504MB )
: EIDE disk on an old IDE controller with FreeBSD?

It should work fine; when the FreeBSD kernel loads, it should probe
the disk and tell you how large it thinks it is. This should be the
full disk capacity, regardless of that the BIOS thinks. One thing you
may have to do if you share this disk with with other OSes is tell
the partition editor in the FreeBSD installer to use a specific
geometry; there's a small chance it may guess the geometry wrong.
(If it does get the geometry wrong, you may get a 'Missing operating
system error' when you boot. Note it is still possible to boot FreeBSD
off your hard disk using the install floppy's boot block; just type
wd(0,a)/kernel at the Boot: prompt.)

The main thing to be concerned about is where you place the root
filesystem (/). While the kernel's protected-mode controller driver
doesn't care about how many cylinders the your disk has, your BIOS
does, and the boot block uses the BIOS to load the kernel image into
memory at boot time. Usually the BIOS can't read past the 1024th
cylinder, so you _must_ make sure that your root filesystem lies
entirely below the 1024th cylinder. And I mean all of it; don't make
one giant filesystem that covers the whole disk and say "Well, it falls
partly below the 1024th cylinder; that should be good enough." It's not
good enough: if you install a new kernel image, you aren't guaranteed
that it all the blocks allocated to it will fall within the first
1024 cylinders, _unless_ the root filesystem itself resides completely
within them.

That said, as long as you follow this rule with the root filesystem, you
can lay out all your other filesystems any way you like.

-Bill

--
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-Bill Paul            (212) 854-6020 | System Manager, Master of Unix-Fu
Work:         wpaul@ctr.columbia.edu | Center for Telecommunications Research
Home:  wpaul@skynet.ctr.columbia.edu | Columbia University, New York City
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