Frequently asked questions -- Debian mdadm
==========================================

Also see /usr/share/doc/mdadm/README.recipes.gz

1. How do I change the preferred minor of a RAID array?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  See item 12 in /usr/share/doc/mdadm/README.recipes.gz and read the mdadm
  manpage (search for 'preferred').

2. How does mdadm determine which /dev/mdX or /dev/md/X to use?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The logic used by mdadm to determine the device node name in the mdadm
  --examine output (which is used to generate mdadm.conf) depends on several
  factors. Here's how mdadm determines it:

  It first checks the superblock version of a given array (or each array in
  turn when iterating all of them). Run

    mdadm --detail /dev/mdX | sed -ne 's,.*Version : ,,p'

  to determine the superblock version of a running array, or
  
    mdadm --examine /dev/sdXY | sed -ne 's,.*Version : ,,p'

  to determine the superblock version from a component device of an array.
  
  Version 0 superblocks (00.90.XX)
  ''''''''''''''''''''''''''''''''
    You need to know the preferred minor number stored in the superblock,
    so run either of

      mdadm --detail /dev/mdX | sed -ne 's,.*Preferred Minor : ,,p'
      mdadm --examine /dev/sdXY | sed -ne 's,.*Preferred Minor : ,,p'

    Let's call the resulting number MINOR. Also see FAQ 1 further up.

    Given MINOR, mdadm will output /dev/md<MINOR> if the device node
    /dev/md<MINOR> exists.
    Otherwise, it outputs /dev/md/<MINOR>

  Version 1 superblocks (01.XX.XX)
  ''''''''''''''''''''''''''''''''
    Version 1 superblocks actually seem to ignore preferred minors and instead
    use the value of the name field in the superblock. Unless specified
    explicitly during creation (-N|--name) the name is determined from the
    device name used, using the following regexp: 's,/dev/md/?(.*),$1,', thus:

      /dev/md0     -> 0
      /dev/md/0    -> 0
      /dev/md_d0   -> _d0
      /dev/md/d0   -> d0
      /dev/md/name -> name
      (/dev/name does not seem to work)

    mdadm will append the name to '/dev/md/', so it will always output device
    names under the /dev/md/ directory.

    If you want to change the name, you can do so during assembly:

      mdadm -A -U name -N newname /dev/mdX /dev/sd[abc]X

    I know this all sounds inconsistent and upstream has some work to do.
    We're on it.

 -- martin f. krafft <madduck@debian.org>  Wed, 02 Aug 2006 16:38:29 +0100

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