Bug#837964: 95a05b3 broke mdadm --add on my superblock 1.0 array

[Anthony DeRobertis]

(please cc me, I'm not subscribed.)

mdadm 3.4 can not manage to add a spare to my array, it fails like:

   # mdadm -a /dev/md/pv0 /dev/sdc3
   mdadm: add new device failed for /dev/sdc3 as 8: Invalid argument

and the kernel logs:

   md: sdc3 does not have a valid v1.0 superblock, not importing!
   md: md_import_device returned -22

This worked in 3.3.4. I performed two git bisects and found that:

a) it was broken by 95a05b37e8eb2bc0803b1a0298fce6adc60eff16
b) it is sort-of fixed by 81306e021ebdcc4baef866da82d25c3f0a415d2d
   (which AFAIK isn't yet released)

I say sort of fixed in that it adds it to the array, but spits out some
worrying errors (and I have no idea if it'd actually work, e.g., if it'd
assemble again):

   # ./mdadm -a /dev/md/pv0 /dev/sdc3 
   mdadm: Warning: cluster md only works with superblock 1.2
   mdadm: Failed to write metadata to /dev/sdc3

I'm not using (or at least not on purpose!) cluster md, as these are
internal SATA drives accessible by only one machine.

I wasn't able to reproduce it on a new (small) test array, so it might
be something specific to this array.

I've reported this bug to Debian, and that bug report contains a lot of
system information that I won't repeat here (because it's quite long):

	https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=837964

that contains mdadm -E output, the mdadm.conf, etc.