RAID1 arrays not starting when drive is missing

I have installed LVM-over-RAID1 on a debian-derived system. Kernel = 2.6.22
mdadm = 2.6.2
lvm2 = 2.02.26

I have two RAID1 devices -- /dev/md0 which is /boot (and does NOT use LVM) and is made up of /dev/sda2 and /dev/sdb2, and /dev/md1 which has LVM (and the rest of the system) over it and is made up of /dev/sda3 and /dev/sdb3.

I've grub'd both /dev/sda and /dev/sdb, and as long as both drives are plugged in, I can boot from either drive and everything works and is happy.

However...

When I try to boot with a drive removed (for testing purposes), grub comes up fine and the system begins to boot, but it appears that the arrays will not start, which means there's no root filesystem available and everything grinds to a halt. This doesn't seem right. Given the nature of RAID1, the arrays darn well should start up even with a missing drive.

The boot output when the drive is missing has:

md: md0 stopped
md: bind 
md: md1 stopped
md: md0 stopped
md: unbind 
md: export_rdev(sda2)
md: bind 
md: md1 stopped
md: bind 

When both drives are present, this section goes:

md: md0 stopped.
md: unbind
md: export_rdev(sdb2)
md: bind
md: bind

One thing I've discovered is that if while the system is running in normal mode I do:
mdadm /dev/md0 --fail /dev/sdb2 --remove /dev/sdb2 mdadm /dev/md1 --fail /dev/sdb3 --remove /dev/sdb3 and then shut down the machine and unplug the drive, then I can boot from the remaining drive, and I see a raid1: raid set md0 active with 1 out of 2 mirrors
(etc.)

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So if the drive fails while the system is running, it can be punted from the array and the machine can boot off one drive. That's nice and all, but that doesn't help in the case where the drive fail event causes the machine to crash (say the drive dies and takes down the bus and the machine locks up).

So what am I missing on why the arrays won't start up when I have a drive unplugged (it doesn't matter which drive)?

--
Rich Carreiro


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