[CentOS] Re: Replacing failed software RAID drive

Scott Silva ssilva at sgvwater.com
Wed Oct 10 23:28:21 UTC 2007

on 10/10/2007 3:36 PM Hugh E Cruickshank spake the following:
> From: Scott Silva Sent: October 10, 2007 15:15
>> on 10/10/2007 2:46 PM Hugh E Cruickshank spake the following:
>>> From: Les Mikesell
>>>> Hugh E Cruickshank wrote:
>>>>> From: Les Mikesell Sent: October 7, 2007 18:53
>>>>>> Then reinstall grub on the drive.
>>>>> Now I have some questions:
>>>>> 1. Since the /boot partition was mirrored and will be restored on the
>>>>>    new sda drive I do not really want to do a full grub install. From
>>>>>    what I have read that will overwrite existing /boot/grub/grub.conf
>>>>>    file. So I just want to write the MBR on the drive. How to I do
>>>>>    that (the docs I have found were rather unclear on that aspect)?
>>>>> 2. Can a install grub on the replaced boot drive with the system still
>>>>>    running?
>>>> Yes, after the /boot partition re-sync has completed, execute grub and:
>>>> root (hd0,0)
>>>> setup (hd0)
>>>> quit
>>>> If you aren't able to keep the system running while doing the swap, you
>>>> can also do this from the rescue mode boot, but you should have the
>>>> contents on the /boot partition first.
>>> Sorry to be obtuse here but I just want to make very sure of what I am
>>> doing before I do it. Will the "setup" command only write the MBR?
>>> >From my reading the GNU GRUB manual I got the distinct impression that
>>> the "setup" command will also write/rewrite the /boot partition (which
>>> I would like to avoid). The documentation for the setup command states:
>>>     Set up the installation of GRUB automatically. This command uses
>>>     the more flexible command install (see Section 13.3.18 [install],
>>>     page 44) in the backend and installs Chapter 13: The list of
>>>     available commands 49 GRUB into the device install device. If
>>>     image device is specified, then find the GRUB images (see Chapter
>>>     10 [Images], page 29) in the device image device, otherwise use the
>>>     current root device, which can be set by the command root. If
>>>     install device is a hard disk, then embed a Stage 1.5 in the disk
>>>     if possible.
>>>     The option �--prefix� specifies the directory under which GRUB
>>>     images are put. If it is not specified, GRUB automatically
>>>     searches them in �/boot/grub� and �/grub�.
>>>     The options �--force-lba� and �--stage2� are just passed to install
>>>     if specified. See Section 13.3.18 [install], page 44, for more
>>>     information.
>>> The second paragraph tends to imply that /boot/grub will be written
>>> to by default and I do not think that this is what I want to happen.
That paragraph should read stored not put. It assumes that the grub files 
needed are already there.
The commands below work. grub-install and other variants are shell scripts to 
accomplish the same thing.
>>> Thanks again for everyone's input (especially Les). It is greatly
>>> appreciated.
>> What "setup" command?
> That would be the grub setup command.
>> You boot from a rescue disk if your system isn't running already.
>> If booting from rescue disk chroot to your installed files.
>> execute grub
>> at its prompt run the following;
>> root (hd0,0)
>> setup (hd0)
>> quit
>> That should let you boot into your stuff.
> I know that will let me boot but that was not what I was asking about.
> As per the documentation I quoted it appears that the grub setup
> command will overwrite the /boot partition that has been restored from
> the mirror drive. As far as I can see that is not desirable. All I want
> to do is install the GRUB MBR and leave the contents of the /boot
> partition intact.
> My question is basically will the grub setup command only install the
> MBR or will it also overwrite the /boot partition as well?
> Regards, Hugh

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