更新1 (2012/12/30)：这个 bug 已经被上报了数个月，直到前几天才有人找到了原因，并放出了 patch。

更新2 (2013/02/09)：这个 bug 的 patch 终于被接受了，这效率……

今天（半个月前）mdadm升级，结果发现我的RAID 5阵列变成只读了，怎么弄也无法激活，就像这样：

localhost biergaizi # umount /dev/md126p3
localhost biergaizi # fsck /dev/md126p3
# 无缘无故变成写保护了
fsck from util-linux 2.21
e2fsck 1.42.6 (21-Sep-2012)
fsck.ext4: Operation not permitted while trying to open /dev/md126p3
You must have r/w access to the filesystem or be root

localhost biergaizi # mdadm --readwrite /dev/md126
# 设置成读写也不起作用
localhost biergaizi # cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md126 : active (read-only) raid5 sdc[2] sdb[1] sdd[0]
976768000 blocks super external:/md127/0 level 5, 64k chunk, algorithm 0 [3/3] [UUU]

md127 : inactive sdb[2](S) sdc[1](S) sdd[0](S)
7356 blocks super external:imsm

unused devices: <none>

# 停掉RAID看看
localhost biergaizi # umount /dev/md126p*
localhost biergaizi # mdadm -S /dev/md126
mdadm: stopped /dev/md126
localhost biergaizi # mdadm -S /dev/md127
mdadm: stopped /dev/md127

# 哦？
localhost biergaizi # mdadm --assemble --scan
mdadm: Container /dev/md/imsm0 has been assembled with 3 drives
mdadm: Started /dev/md/Volume0_0 with 3 devices

localhost biergaizi # cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md126 : active (auto-read-only) raid5 sdc[2] sdb[1] sdd[0]
976768000 blocks super external:/md127/0 level 5, 64k chunk, algorithm 0 [3/3] [UUU]

md127 : inactive sdb[2](S) sdc[1](S) sdd[0](S)
7356 blocks super external:imsm

unused devices: <none>

只能把这个软件包也mask掉。