Re: Grub Manual ... Solved
- From: Gene Heskett <gene.heskett@xxxxxxxxxxx>
- Date: Sat, 20 Oct 2007 13:47:00 -0400
On Saturday 20 October 2007, Les Mikesell wrote:
Gene Heskett wrote:
That is this line in a verse of grub.conf:
root (hd0,0)
which defines that its the drive normally set for master on the IDE0
cable,
Not necessarily...
the one on the end of the cable, thats the hd0 portion, and its the first
partition on that drive, which is the second 0 since all this crap runs on
a base zero numbering system.
Any resemblance between a Linux hd? device and a grub hd? is purely
coincidental. In Linux the hd?'s are strictly IDE (and they are going
away). In grub, the hd?'s refer to the bios bootable devices. If, for
example, you have scsi devices and have configured your motherboard bios
to boot from them, grub's hd0 will be your booting scsi drive even if
you also happen to have an IDE present that linux will see as /dev/hda.
I didn't know that, not having crossed that bridge, thanks.
It could just as easily say root (hd3,2) to
indicate that it would be 3rd partition in, of the drive set as slave on
the IDE1 cable, that is just as valid. I haven't tried it set for an
'extended' partition, and I'd assume it would take a smarter than the
average bear bios to handle that.
Many (most?) motherboards only see 2 disks in the boot process, although
they may let you select from a larger number of choices which order to
try. So you may have trouble using anything higher than hd1 - which
again refers to what bios is using, not a particular ide position. And
you may have non-bootable disks that have no bios driver at all, but
which linux will use normally.
I have been under the impression that if the bios scan found them, they were
in fact usable, is this incorrect?
This is the usage that I would have changed, were I the one who wrote
grub, from root to boot, because it actually tells grub where its boot
files are.
Grub has to know the bios notation, since that is all it can use to
access the files until the kernel and driver modules are installed.
Its second usage, the argument in the kernel string, tells the kernel, in
very similar drive & partition syntax, where the partition to be mounted
as '/' is.
That is this line in a given grub verse:
kernel /vmlinuz-2.6.23 ro root=/dev/VolGroup00/LogVol00 rhgb
And this could just as easily say root=/dev/hda7 since by this time, the
kernel is initializing and knows about extended partitions by then. This
to me is the correct usage of the word 'root' because it truly is the root
of all the filesystems mounted.
But at this point the kernel is running, has loaded its device drivers,
has access to all devices, even ones bios doesn't handle, and this
reference must be in linux terms where, for example /dev/hda7 does in
fact refer to a disk with a particular IDE controller and selection.
Even if you booted from a scsi drive that bios saw as the 0x80 device
and grub called hd0, the kernel can mount a partition on /hda (an IDE
device) as the root filesystem.
But I didn't write grub, so we're stuck with the double usage of 'root'
and the confusion factor it creates for newbies.
They are double because they are entirely different things.
That part is still confusing to the newbie though.
AIUI, grub, if not dealing with a LVM system, can function without a /boot
partition, in which case the above syntax must be modified, and the grub
installer does this, to gave a full path like this:
kernel /boot/vmlinuz-2.6.23 ro root=/dev/hda1
(a base 1 numbering scheme is used here, furthering the newbie confusion
quotient, that is equ to (hd0,0) in the other syntax)
One is linux, one is a peculiar adaptation of a bios concept.
assuming the /boot directory is in fact available in the drive & partition
specified by the first usage above, but its not an optional setup I've
ever used and I go back to RH5.0.
On older machines (circa RH5 era...) PC bios was limited to accessing
1024 cylinders for some time after larger drives were available. If the
kernel and initrd files were in the filesystem in a location beyond that
limit, the boot loader would be unable to access them to complete the
boot. Thus the installer always tries to make boot a small partition
and first on the disk to avoid any problem. The bios addressing scheme
has been modified to work around the limits in current machines.
Now, I suspect that you'll probably argue with me, but it will be a one
sided argument since I tend not to argue about points I know about after
having stated them once.
I'll argue because you've stated something wrong or at least misleading.
--
Les Mikesell
lesmikesell@xxxxxxxxx
Thanks for those bios related clarifications. I haven't tried to diddle the
order in the bios, so I was unaware of that until now as it hadn't crossed my
radar.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The number of licorice gumballs you get out of a gumball machine
increases in direct proportion to how much you hate licorice.
--
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