On Fri, 29 Oct 2010 15:18:09 +0100
Mathias Burén [off-list ref] wrote:

Question 3: Before I created this RAID5 array I did a quick RAID0 test
array just for fun, using 2 full devices (not partitions). Now I have
this:

mdadm  --examine --verbose --scan
ARRAY /dev/md/raid0-test level=raid0 metadata=1.2 num-devices=2
UUID=b84cc081:1ae27b49:d5ae466c:377ba300 name=ion:raid0-test
   devices=/dev/sdf,/dev/sdb
ARRAY /dev/md/0 level=raid5 metadata=1.2 num-devices=6
UUID=e6595c64:b3ae90b3:f01133ac:3f402d20 name=ion:0
   devices=/dev/sdg1,/dev/sdf1,/dev/sde1,/dev/sdd1,/dev/sdc1,/dev/sdb1

Is it safe to erase the raid0-test superblocks on device /dev/sdb and
/dev/sdc or will it interfere with my RAID5 array (which is lying on
top of partitions) ?

It should be safe to
  mdadm --zero-superblock /dev/sdf /dev/sdb

As you have 1.2 metadata, that info will be 4K from the start of the device.
Depending on how you partitioned the devices, that is either in dead space
between the partition table and the first partition, or it is in dead space
in the first partition just before the md metadata.

So the old metadata is still visible, creating the new arrays clearly didn't
over-write it, so they don't really care what is there...

That statement isn't 100% general. A block in the data area of the new array
could be unchanged by creating an array, yet changing it could still corrupt
parity.  However you can be certain that the metadata for a whole-device
array does not lie in the data area for a partitioned array of the same
metadata type.

NeilBrown
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