Replacing disk in Linux Software RAID 1

Robin Laing Robin.Laing at drdc-rddc.gc.ca
Wed Aug 18 15:54:00 UTC 2004


jludwig wrote:
> On Wed, 2004-08-18 at 10:19, Robin Laing wrote:
> 
>>jludwig wrote:
>>
>>>On Tue, 2004-08-17 at 11:26, Robin Laing wrote:
>>>
>>>
>>>>Michael Mansour wrote:
>>>>
>>>>
>>>>>Hi Robin,
>>>>>
>>>>>
>>>>>
>>>>>>Would it be possible to write the MBR to the second
>>>>>>disk just in case?
>>>>>>-- 
>>>>>>Robin Laing
>>>>>
>>>>>
>>>>>Yes, I've tested this by removing the first drive and
>>>>>writing grub to the MBR of the second drive and it did
>>>>>work fine, although when putting the first drive back
>>>>>in there then I had the issue with two MBR's which got
>>>>>things a little confused.
>>>>>
>>>>>Basically, what I learnt from that saga is that grub
>>>>>should only reside on one drive.
>>>>>
>>>>>Michael.
>>>>>
>>>>
>>>>I for one when I was a system admin, did not like getting called in at 
>>>>03:00 due to a crash.  And doesn't always seem to happen at 03:00 when 
>>>>things go bad?  :)
>>>>
>>>>I feel it should be there.  Now if you do write it to the second disk 
>>>>and you have problems, this could be a problem.
>>>>
>>>>-- 
>>>>Robin Laing
>>>
>>>Many newer BIOS' will allow for booting off of a secondary drive.
>>>
>>>If this is the case with your machine I would just load the second drive
>>>where it sits and just change the BIOS setting if necessary.
>>>
>>>(I used to do this with SCSI and IDE for a duel boot system.)
>>>
>>
>>This isn't the issue.
>>
>>As stated by Michael, having Grub on the second drive caused problems. 
>>  My point was in a situation where your /boot is actually part of a 
>>Raid array, and the second drive "cannot have Grub on it" then it 
>>isn't much use in an emergency as you will have to have a copy of the 
>>MBR to update your second drive.
>>
>>Is this an issue with RAID or Grub or the Bios?
>>
>>-- 
>>Robin Laing
> 
> If I remember correctly grub can be on both and chain loaded, but, if
> your first drive has issues and can't start grub you're dead.

I think Nigel Wade made a good point that LILO is RAID aware while 
GRUB isn't.

If GRUB is going to be the default installation, it should be made 
RAID aware.  Maybe with a menu option to select a different drive in a 
recovery mode?

I do agree that changing the drive is one way around the problem but 
Michael stated that he had problems when the MBR was mirrored on both 
drives.  I deleted his latest message so I cannot refer to it.

As stated earlier.  I don't use RAID for my boot drive but others do 
and this is/could be an issue for them.  He did state that we was 
going to go to hardware raid.

-- 
Robin Laing





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