Bug#614841: mdadm --create should provide an option to set a custom data offset for metadata > 1.0

[Daniel Kahn Gillmor]

Package: mdadm
Version: 3.1.4-1+8efb9d1
Severity: wishlist

mdadm --create seems to default to the new 1.2 metadata format, which
is found 4KiB in from the start of the component device.

As a result, the actual data is stored at a non-zero offset from the
start of each component.

Some block devices these days have different optimal block boundaries
(e.g. see the recent LWN article on flash translation layers [0] --
some have 4MiB eraseblocks).

It would be good to be able to specify an arbitrary size for the data
offset for metadata 1.1 and 1.2 to ensure that underlying block device
access is aligned with sensible lower-layer boundaries.

if you have a spare block device to play with, you can try making a
RAID array to see what happens:

MD_NEW=/dev/md1
COMPONENT_SPARE=/dev/xda7
mdadm $MD_NEW --create -l 1 -n 2 $COMPONENT_SPARE missing
mdadm --stop $MD_NEW
mdadm --examine $COMPONENT_SPARE

I get a Data Offset of 24 sectors, and i see no way of making that
come out different.

It seems other people have asked this before [1], but i see no clear
way of pursuing it, other than forcing metadata 1.0 or 0.90.

Feel free to forward this request upstream -- i'm not sure where it
would be most useful, but i thought i'd document it as a wishlist bug
for debian.

    --dkg

[0] https://lwn.net/Articles/428584/
[1] http://marc.info/?l=linux-raid&m=126263344005002&w=2

-- System Information:
Debian Release: wheezy/sid
  APT prefers testing
  APT policy: (500, 'testing'), (200, 'unstable'), (1, 'experimental')
Architecture: i386 (i686)

Kernel: Linux 2.6.37-1-686 (SMP w/1 CPU core)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages mdadm depends on:
ii  debconf                       1.5.38     Debian configuration management sy
ii  libc6                         2.11.2-11  Embedded GNU C Library: Shared lib
ii  lsb-base                      3.2-27     Linux Standard Base 3.2 init scrip
ii  makedev                       2.3.1-89   creates device files in /dev
ii  udev                          166-1      /dev/ and hotplug management daemo

Versions of packages mdadm recommends:
ii  module-init-tools             3.12-1     tools for managing Linux kernel mo
ii  postfix [mail-transport-agent 2.7.1-1    High-performance mail transport ag

mdadm suggests no packages.

-- debconf information excluded