[CentOS] Re: raid1 disk format?

Les Mikesell lesmikesell at gmail.com
Wed Jun 11 17:41:40 UTC 2008

Ross S. W. Walker wrote:
>>>>> If you have a disk with several partitions set up as members of a 
>>>>> raid1 md devices, can you make a dd image of that disk to replace its 
>>>>> matching drive with identical partitions or are there differences 
>>>>> between the mirrored partitions?
>>>> you can 'dd' the MBR and then re-add the partitions to the raid for
>>>> resyncing with 'mdadm'.
>>>> # dd if=/dev/sda of=/dev/sdb bs=512 count=1
>>>> # mdadm /dev/md0 --add /dev/sdb1
>>>> # mdadm /dev/md1 --add /dev/sdb2
>>>> If you want to really make sure you got everything you could dd the
>>>> whole first track with:
>>>> # dd if=/dev/sda of=/dev/sdb bs=512 count=63
>>>> -Ross
>>> Or sfdisk -d /dev/sdX | sfdisk /dev/sdY
>>> where x is source and y is the target.
>>> This will work across drives that have slight geometry differences.
>> What I was hoping to do was to take the grub setup, the partitioning 
>> info and the contents in one shot and have the disks pair automatically 
>> when booted.  They didn't - but I think the other parts worked.
>> Now, is there a way to change the uuid on a running raid1 set?  I'd 
>> prefer that if the split and re-paired disks ever find their way back to 
>> the same machine that they not sync again.
> 'mdadm' writes a listing of the devices in the array to the md
> superblock and orders them by number,major,minor. You cannot add
> another device to the array with the same tuple.

Isn't this updated at detect time so the device minor's should always be 

> If you dd the first sector of the drive though you will duplicate
> the partition table and grub boot loader to the other drive. Then
> md-device-mapper will take care of copying the data over.

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.

   Les Mikesell
    lesmikesell at gmail.com

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