Re: Grub Manual ... Solved

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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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