Kurt Tunkko wrote:
> Hello Holker,
>
> Holger Parplies wrote:
>
>   
>>> 3. Mirror partition tables from one of the existing disks:
>>>
>>>     # sudo sfdisk -d /dev/sda | sfdisk /dev/sdc
>>>       
>> apart from something having been mangled ("???"), I tend to wonder why you
>> need root permission to read the partition table of /dev/sda but not to write
>> it to /dev/sdc ;-). You might, of course, have relaxed the permissions on
>> /dev/sdc, but I'd spare an extra 'sudo' for the howto ;-).
>>     
>
> I took the information above from the page:
>
> Setting up software RAID in Ubuntu Server
> http://advosys.ca/viewpoints/2007/04/setting-up-software-raid-in-ubuntu-server/
>
> " Once system as been rebooted with the new unformatted replacement 
> drive in place, some manual intervention is required to partition the 
> drive and add it to the RAID array.
> The new drive must have an identical (or nearly identical) partition 
> table to the other. You can use fdisk to manually create a partition 
> table on the new drive identical to the table of the other, or if both 
> drives are identical you can use the “sfdisk” command to duplicate the 
> partition. For example, to copy the partition table from the second 
> drive “sdb” onto the first drive “sda”, the sfdisk command is as follows:
>
> sfdisk –d /dev/sdb | sfdisk /dev/sda"
>
> I don't know if it's possible to add a 3rd drive to the RAID, that 
> hasn't got the right partitions on it :-?
>
>   
>> I believe the original idea is *not* to temporarily cripple your RAID but
>> rather to add a third disk (three way mirror).
>>     
>
> you're right I changed my setup and have now a three way raid, so that I 
> can unplugg one drive and keep it as offline backup.
>
>  > I'm not sure if you can do that after initial creation of the array,
>  > but the man page suggests it should be possible "on kernels which
>  > provide necessary support".
>
> On Ubuntu Server I was able to add a third harddisk to the array and get 
> it synced. After sync had been completed I can remove the 3rd drive from 
> the array and lock it away for offline storage.
>
>    Change a 2-disk RAID1 to a 3-disk RAID:
>    # sudo mdadm --grow --raid-devices=3 /dev/md0
>
>    Add a 3rd drive to the existing 2disk RAID1:
>    # sudo mdadm --add /dev/md0 /dev/sdc1
>    -> spare will be rebuilded
>
>    Remove 3rd disk from RAID
>    # sudo mdadm --fail /dev/md0 /dev/sdb1
>    # sudo mdadm --remove /dev/md0 /dev/sdb1
>
> While this approach is much better than the one I suggested yesterday it 
> still leads to some Questions:
>
> 1) Is there a way to add the 3rd drive to RAID1 as soon as it will be 
> connected to the system (External harddrive that is oonnected via usb2)?
> Or more generally: Can I run a script when an external storage device is 
> connected via usb?
>   

Yes.  http://ubuntuforums.org/showthread.php?t=502864

> 2) Do I need to resize my RAID after removing the 3rd harddrive with
>     # sudo mdadm --grow --raid-devices=2 /dev/md0
>   

Personally, I wouldn't bother, unless it made my monitoring software 
throw a false error.

>     Are there any problems when the RAID will be used in clean, but 
> degraded state?
>   

I doubt it.  The mirror is going to be active with two drives, so writes 
might even be faster.

> - Kurt
>   

Chris

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