[CentOS] Re: raid1 disk format?

Scott Silva ssilva at sgvwater.com
Wed Jun 11 18:06:12 UTC 2008

on 6-11-2008 10:41 AM Les Mikesell spake the following:
> 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 
> different?
>> 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.
Are they dd copies of the different nodes (IE.. dd copy of sda paired with dd 
copy of sdb)?

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