[CentOS] Re: raid1 disk format?

Les Mikesell lesmikesell at gmail.com
Thu Jun 12 18:24:13 UTC 2008

Scott Silva wrote:
>>>>>> I'm curious as to why 2 complete dd'd copies don't pair at boot.  
>>>>>> One comes up running and it does work to mdadm --add the partner 
>>>>>> partitions and after the resync they do automatically pair at boot.
>>>>> Are they dd copies of the different nodes (IE.. dd copy of sda 
>>>>> paired with dd copy of sdb)?
>>>> No - what was sda was dd'd twice with the dups put in sda and sdb.
>>> That is why it didn't work. The superblock knows which drive it was 
>>> on, and knows if there are dupes.
>> But I thought the locations were re-detected at boot/assembly time.
>> The other question is whether it is possible to change the uuid while 
>> the system is running or if it would have to be done from a CD boot. 
>> I've cloned several machines from one initial setup and if copies of 
>> the disks ever find their way back into one box I'd prefer not to have 
>> the wrong set try to re-sync.
> The locations can re-detect, but I still think it will stop if it 
> detects 2 identical superblocks.
> I don't think you can change the UUID of a running array, and I'm not 
> sure if you can do it easily on a stopped array. I think the safest 
> thing is to use the --add to join the pair. Since they are dd clones, 
> the sync should be fairly fast. Just be careful.

Yes, the sync does work fine and once finished the set will always start 

> I know that it seems like an easy way to clone machines, but I think 
> there are better and safer ways to clone machines.

You can't beat dd for getting everything exactly the same regardless of 
what you changed - or just splitting the mirrors and letting each sync 
to new partners but then you have to reinstall grub.  I prefer 
clonezilla for non-raid configurations but most of the machines I care 
about are configured with raid1.

   Les Mikesell
    lesmikesell at gmail.com

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