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Old 04-05-2008, 04:54 AM
Les Mikesell
 
Default What will happen? Linux software raid autodetecting on an old array...

Matthew Miller wrote:

Okay, so, here's the situation. Motherboard on my old computer (a Shuttle)
went out. Decided to get a new computer, because hey, it was four years old.

New computer came with two new SATA drives, which I've set up to to be
mirrored with software raid -- partitions /dev/md0 and /dev/md1.

All is good.

Now, had the exact same disk configuration in my old system. If plug in
those disks into the 3rd and 4th SATA ports, and boot, what will happen?

I hope the RAID autodetect system is smart enough to not get the old
/dev/md{0,1} mixed up with the new one and eat data. Right?

What Will Happen?


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.


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Les Mikesell
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Old 04-06-2008, 02:46 PM
Les Mikesell
 
Default What will happen? Linux software raid autodetecting on an old array...

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.


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Les Mikesell
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