Please explain status/background of /dev/mdX versus /dev/md/X

[Frans Pop]

(Please reply to debian-boot; reply-to set.)

Hi Martin,

Over the past few weeks I've solved several issues in D-I related to the use 
of /dev/mdX versus /dev/md/X and I've been wondering what the official 
status is of both as it seems there are at least some inconsistencies.

Partman currently prefers the use of /dev/md/X and uses that when creating 
new RAID devices. This results in both /dev/md/X and /dev/mdX block device 
files being created.

I've been working on the assumption that /dev/md/X is the "newer" form and 
that the intention is to transition to that. Is that correct?

Here are some of the inconsistencies I've noticed:
- /dev/mdX and /dev/md/X are created with different permissions
- if a new RAID device is created using /dev/mdX, the "new" block device
  files are _not_ created
- 'mdadm --examine --scan --config=partitions' outputs the "old" block
  device names which again means that the "new" block devices are not
  created if that is used as input to assemble existing RAID devices

Especially the last issue affected partman when the system being installed 
had a pre-existing RAID partition setup. I've now worked around that in 
mdcfg by converting old to new using sed before doing the assemble.

Could you provide some insight into what the current status is from an mdadm 
(and maybe kernel) upstream PoV and how the transition is expected to 
proceed?

TIA,
Frans
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