What will happen? Linux software raid autodetecting on an old array...

[Les Mikesell]

Matthew Miller wrote:
> On Fri, Apr 04, 2008 at 11:54:55PM -0500, Les Mikesell wrote:
>> I think it will pick one of each of the mdN devices and ignore the other.  
>> It won't eat data unless the mismatched disks have matching UUIDs which is 
>> unlikely unless you image-copied them or had synced with the other set in 
>> the past.  If you want it to ignore the new set until you reconfigure it, 
>> fdisk the partition types to something other than FD so it won't autodetect 
>> at boot up.
> 
> Okay, thanks. Any suggestions on how to reconfigure it to use higher numbers
> on the fly without losing data? I'd like the old md0/md1 to end up as
> md2/md3.

If you have a place to hold a backup you can mount one of the underlying 
partitions and rsync it somewhere for safekeeping.  I've always rebuilt 
from scratch when moving mirrors but I think you can use mdadm 
--assemble or --create to put them together.  A safe approach might be 
to use --create to make the new device but only specify one of the 
partitions with the other as --missing.  Then if the new md device still 
has the data, you can mdadm --add the other partition, if it has a 
problem you can mount the other partition and copy it over.  Even these 
operations are slightly dangerous, though since a disk error on either 
partition will break things.

-- 
   Les Mikesell
    lesmikesell at gmail.com