update-grub help

[Tom H]

On Wed, May 26, 2010 at 11:37 PM, Goh Lip <g.lip at gmx.com> wrote:
> On 05/27/2010 11:03 AM, Tom H wrote:
>> On Wed, May 26, 2010 at 10:48 PM, Goh Lip<g.lip at gmx.com>  wrote:
>>> On 05/27/2010 03:23 AM, Goh Lip wrote:
>>>
>>>> Re; use of "search xxxxx --set xxx " instead of "set root=(hdx,y)
>>>> and /dev/sdxy" : trying to get you some definitive  information on this,
>>>> but so far, most of this is "inferred"  through the grub developers
>>>> mailing list and of course my own experience in failure to boot due to
>>>> 'movement' in hdx,y and /dev/sdc or /dev/sdd, particularly in grub
>>>> legacy. Will post to you if I get more information. But I am reluctant
>>>> to send info that is not indisputatble.
>>>
>>> Tom, refer https://help.ubuntu.com/community/UsingUUID
>>>/
>>> First line writes:
>>>
>>> Linux now prefers to use UUID (Universally Unique Identifier), LABEL, or
>>> symlinks to identify media storage devices on a system. Directly using
>>> /dev/hd*# or /dev/sd*# is no longer preferred since these device
>>> assignments can change between system boots:
>>
>> I am not advocating the use of /dev/sdXY, far from it.
>>
>> The files in /dev/disk/by-id and /dev/disk/by-uuid are unique symlinks
>> to /dev/sdXY and are created by udev.
>>
>> Instead of using "root=UUID=<uuid>", you can use
>> "root=/dev/disk/by-uuid/<uuid>" to refer in as uniquely a way to the
>> same partition.
>>
>> It looks like OpenSuse uses "root=/dev/disk/by-id/<id>" (maybe there
>> isn't a "root=ID=<id>" option) and it is just as valid and unique a
>> way as a uuid.
>
> Yes, understood. Got it. But when Dave deletes the "search xxxxxx set
> xxx", this means using the set root=(hdx,y) which is unreliable and will
> set that as the root. Even if he specify kernel correctly,as in "linux
> /boot/vmlinuz-xxx xxxxx root=/dev/disk/by-id/xxxxxx", his "root
> partition" may be wrong and will not boot.

If the "set root=(hdx,y)" is unreliable, then the "search ..." is
unreliable too because it only sets a uuid to the first partition that
it finds, which is, most probably, (hdx,y), and corresponds to / (or
/boot if it is a separate partition). The correspondence between
partitions and their grub (hdx,y) name must be less volatile than that
between partitions and their corresponding udev /dev/sdXY name...