slow boot (exe=fsck) with root partition on md

Hello,

This was initialy asked as a question, which get answered with a suggestion to report a bug.

My system is currently slow to boot (bootchart reports about 70s in exe, between mdadm+udevadm and udev+udevadm : e.g. from start of record, 5.5s to reach end of modprobe, then small wait-for-root, mdadm & udevadm at 6.2s, then exe till 73.1s, then continue booting normally (udev,udevadm,hostname, plymouthd, ureadahead... everything goes in before at 85s)

My root partition(/) is a raid5 on 7 disks, on md0, using ext4fs. (no LVM; no internal partition table)

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdb1[1] sdc1[2] sda1[0] sde1[4] sdf1[5] sdg1[6] sdd1[3]
      11721077760 blocks super 1.2 level 5, 512k chunk, algorithm 2 [7/7] [UUUUUUU]
      bitmap: 2/15 pages [8KB], 65536KB chunk

unused devices: <none>

From a blog on the web ( http://georgovassilis.blogspot.fr/2011/12/exe-process-on-slow-ubuntu-bootchart.html ), it seems due to a failure to stop cleanly the ext4fs on shutdown/reboot.

Am I right to think that in my situation, the mdadm should not be stopped before the umountfs or umountroot is performed ?
(/etc/rc0.d (and rc6.d) lists K25mdadm, S40umountfs and S60umountroot )
Is it safe to remove K25mdadm from runlevel 0 & 6 ?

package mdadm : 3.2.5-1ubuntu0.2

Description: Ubuntu 12.04.1 LTS
Release: 12.04

mdadm:
  Installed: 3.2.5-1ubuntu0.2
  Candidate: 3.2.5-1ubuntu0.2
  Version table:
 *** 3.2.5-1ubuntu0.2 0
        500 http://archive.ubuntu.com/ubuntu/ precise-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     3.2.3-2ubuntu1 0
        500 http://archive.ubuntu.com/ubuntu/ precise/main amd64 Packages

I would expect the system to boot in less than 50 seconds, and especially, without a 1 minute of no screen display but a single flashing "_" (was grub found ? is the bios still checking... plymouth shows up only after the expiration of that 60+ seconds black screen, so if I do turn the head from the screen at boot time, I cannot know)

What happen instead: bios runs, grub runs then disappear and after a very short disk activity (led), nothing seems to happens for at least 60s (I guess it's all in memory-cached i/o past the few seconds).