On Tue, Nov 4, 2014 at 1:06 AM, Michael Tokarev [off-list ref] wrote:

01.11.2014 21:53, Stephen Kent wrote:

quoted

I have set up a machine to use an md array as the root device. I have
created the array with metadata type 1.2, name "root," on host
"optimus" (so the full metadata name is "optimus:root").

I'm using Gentoo Linux (kernel 3.16.5-gentoo), and previously I had no
initramfs so I wrote a short one to assemble the array. My initramfs
sets the hostname and runs "mdadm -As," which correctly auto-assembles
the root device array on /dev/md/root (even without an
/etc/mdadm.conf).

Once the initramfs script is finished, init starts which then starts
udev. Now, at this point the array is already assembled, but the
/dev/md/* name is lost (and so is /run/mdadm/map) since those files
were on the initramfs' tempfs root.

This is your problem.  You should not lose neither /dev nor /run
when switching root.  Usual thing to do is to mount tmpfs /dev and
tmpfs /run in initramfs and do mount --move /dev /root/dev (and
the same for /run) before switching root.

Thanks,

/mjt

Hm, interesting. I didn't realize a mount point could be moved, that's
good to know.

My initramfs is a short custom shell script that I wrote, basically
all it does is assemble the array and mount the root filesystem. I'm
not mounting a tmpfs and/or udev on /run and /dev, so unfortunately
there aren't any mount points to move to the target root mountpoint at
the end of init. /run and /dev are only mounted on my machine after
leaving the initramfs.

Fortunately, NeilBrown's patch works for me which solves the problem
and allows mdadm to work correctly with a minimal initramfs.

Stephen