Bug#517731: Please add raid1 -> raid5 reshaping support

[Goswin Brederlow]

Package: mdadm
Version: 2.6.8-12-gb47dff6-2
Severity: wishlist

Hi,

just recently Ingo Juergensmann brought up a question on irc about
converting a raid1 into a raid5. Currently --grow does not support
that but one can achive it manualy with a little trick.

A prerequisite is that it is a raid1 with 2 disks. If you have more
than take some out and put them aside as backup. In raid5 the parity
is the xor of the data blocks in each stripe. If a stripe only has one
data block and one parity block (2 disk raid5) they are identical,
just like raid1 has them. So the following works:

1) stop the raid
2) mdadm --create -l 5 -n 2 --assume-clean /dev/md0 /dev/sda1  /dev/sdb1

It would be nice if mdadm could mention this in the manpage or, even
better, support this directly with --grow. If the raid1 has more than
2 disks the remaining disks could be made spare disks as first step
and then the raid could be grown as second step.

MfG
	Goswin

-- System Information:
Debian Release: squeeze/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (200, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 2.6.22.2-mrvn
Locale: LANG=C, LC_CTYPE=de_DE (charmap=ISO-8859-1)
Shell: /bin/sh linked to /bin/bash

Versions of packages mdadm depends on:
ii  debconf                       1.5.25     Debian configuration management sy
ii  libc6                         2.9-3      GNU C Library: Shared libraries
ii  lsb-base                      3.2-20     Linux Standard Base 3.2 init scrip
ii  makedev                       2.3.1-88   creates device files in /dev
ii  udev                          0.125-7    /dev/ and hotplug management daemo

Versions of packages mdadm recommends:
ii  exim4-daemon-heavy [mail-tran 4.69-9     Exim MTA (v4) daemon with extended
ii  module-init-tools             3.4-1      tools for managing Linux kernel mo

mdadm suggests no packages.

-- debconf information excluded