CentOS6 initrd says “already mounted or /sysroot busy”

If you are booting a CentOS 6 system after having migrated its root filesystem to a new volume, you might get the following errors if /proc or /sys is missing:

EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts: 
mount: /dev/mapper/vg0-root already mounted or /sysroot busy
mount: according to mtab, /dev/mapper/vg0-root is already mounted on /sysroot
dracut: Remounting /dev/mapper/vg0-root with -o relatime,ro
EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts: 
mount: /dev/mapper/vg0-root already mounted or /sysroot busy
mount: according to mtab, /dev/mapper/vg0-root is already mounted on /sysroot
dracut: Remounting /dev/mapper/vg0-root with -o relatime,ro
EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts: 
dracut Warning: Can't mount root filesystem

To fix this, all you need to do is mount the root filesystem and “mkdir proc/ sys/”. You can even do this from inside of dracut if you add the “rdshell” argument to the end of your kernel command line:

dracut:/# mount -o remount,rw /sysroot
dracut:/# mkdir /sysroot/proc /sysroot/sys
dracut:/# mount -o remount,ro /sysroot
dracut:/# exit
(You may need to reboot the server)

-Eric