mjhjmtu

I would like to have a root file system composed of several aufs branches mounted on /.

My Linux is a custom one created using buildroot and deployed on a board with a SAMA5D2 processor.

The mmc of my board is partionned as follow:

<device> <size> <type> <flag>
/dev/mmcblk0p1 20M FAT32 boot1
/dev/mmcblk0p2 20M FAT32 boot2
/dev/mmcblk0p3 500M ext2 rootfs
/dev/mmcblk0p4 13.2G Extended partition -
/dev/mmcblk0p5 100M ext2 factory_application
/dev/mmcblk0p6 500M ext2 update_overlays1
/dev/mmcblk0p7 500M ext2 update_overlays2
/dev/mmcblk0p8 12.1G ext2 persistant_data

There are two boot partition and two update overlays to be mounted as read/write when an update will be applied, then partitions are switched.

I want the root file system to be layered as follow

upper layer | persistant_data read-write
 | update_overlays read-only
 | factory_application read-only
 | rootfs read-only
lower layer V boot read-only

What I have done so far

So far I have been able to mount partitions at boot and to create a aufs mount point as follow:

Here are kernel parameters

console=ttyS0,115200n8 root=/dev/mmcblk0p3 ro rootwait quiet g_ether.host_addr=00:b0:ce:00:00:02 g_ether.dev_addr=00:b0:ce:00:00:01

Here is fstab

# cat /etc/fstab 
# <file system> <mount pt> <type> <options> <dump> <pass>
/dev/root / ext2 ro,noauto 0 1
proc /proc proc defaults 0 0
devpts /dev/pts devpts defaults,gid=5,mode=620,ptmxmode=0666 0 0
tmpfs /dev/shm tmpfs mode=0777 0 0
tmpfs /tmp tmpfs mode=1777 0 0
tmpfs /run tmpfs mode=0755,nosuid,nodev 0 0
sysfs /sys sysfs defaults 0 0
/dev/mmcblk0p1 /mnt/boot vfat ro 0 0
/dev/mmcblk0p5 /mnt/factory ext2 ro 0 0
/dev/mmcblk0p6 /mnt/update ext2 ro 0 0
/dev/mmcblk0p8 /mnt/pd ext2 defaults 0 0

And here is inittab

# cat /etc/inittab
# /etc/inittab
#
# Copyright (C) 2001 Erik Andersen <andersen@codepoet.org>
#
# Note: BusyBox init doesn't support runlevels. The runlevels field is
# completely ignored by BusyBox init. If you want runlevels, use
# sysvinit.
#
# Format for each entry: <id>:<runlevels>:<action>:<process>
#
# id == tty to run on, or empty for /dev/console
# runlevels == ignored
# action == one of sysinit, respawn, askfirst, wait, and once
# process == program to run

# Startup the system
::sysinit:/bin/mount -t proc proc /proc
::sysinit:/bin/mkdir -p /dev/pts
::sysinit:/bin/mkdir -p /dev/shm
::sysinit:/bin/mount -a
::sysinit:/bin/mount -t aufs -o br=/mnt/pd=rw:/mnt/update=ro:/mnt/factory=ro:/mnt/boot=ro,udba=reval none /mnt/aufs
::sysinit:/bin/hostname -F /etc/hostname
# now run any rc scripts
::sysinit:/etc/init.d/rcS

# Put a getty on the serial port
console::respawn:/sbin/getty -L console 0 vt100 # GENERIC_SERIAL

# Stuff to do for the 3-finger salute
#::ctrlaltdel:/sbin/reboot

# Stuff to do before rebooting
::shutdown:/etc/init.d/rcK
::shutdown:/sbin/swapoff -a
::shutdown:/bin/umount -a -r

As you can see, the aufs mount is working quite well

# mount
/dev/root on / type ext2 (ro,relatime,block_validity,barrier,user_xattr)
devtmpfs on /dev type devtmpfs (rw,relatime,size=222416k,nr_inodes=55604,mode=755)
proc on /proc type proc (rw,relatime)
devpts on /dev/pts type devpts (rw,relatime,gid=5,mode=620,ptmxmode=666)
tmpfs on /dev/shm type tmpfs (rw,relatime,mode=777)
tmpfs on /tmp type tmpfs (rw,relatime)
tmpfs on /run type tmpfs (rw,nosuid,nodev,relatime,mode=755)
sysfs on /sys type sysfs (rw,relatime)
/dev/mmcblk0p1 on /mnt/boot type vfat (ro,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
/dev/mmcblk0p5 on /mnt/factory type ext2 (ro,relatime,block_validity,barrier,user_xattr)
/dev/mmcblk0p6 on /mnt/update type ext2 (ro,relatime,block_validity,barrier,user_xattr)
/dev/mmcblk0p8 on /mnt/pd type ext2 (rw,relatime,block_validity,barrier,user_xattr)
none on /mnt/aufs type aufs (rw,relatime,si=a9f95e05)

And the partitions are united in the folder /mnt/aufs

# ls /mnt/aufs/
boot_file lost+found sama5d2.dtb zImage
factory_file pd_file update_file

The problem I'm facing

What I would like to do is to have the aufs mount into / instead of /mnt/aufs

- First try

However when I'm trying to use / as the mountpoint, the mount command seems to work but I can't see any file from others partitions into / folder

# mount -t aufs -o br=/mnt/pd=rw:/mnt/update=ro:/mnt/factory=ro:/mnt/boot=ro,udba=reval none /

# mount
[...]
none on / type aufs (rw,relatime,si=9ef5aaee)

Files from the different partitions aren't present into /

# ls /
bin etc lib linuxrc mnt proc run sys usr
dev home lib32 media opt root sbin tmp var

- Second try

I have also tried to mount / as a layer into /mnt/aufs
which worked well.

# mount -t aufs -o br=/mnt/pd=rw:/mnt/update=ro:/mnt/factory=ro:/=ro:/mnt/boot=ro,udba=reval none /mnt/aufs

# mount
[...]
none on /mnt/aufs type aufs (rw,relatime,si=73ed3d1d)

Now files from the different partitions are present into /mnt/aufs

# ls /mnt/aufs/
bin home media root tmp
boot_file lib mnt run update_file
dev lib32 opt sama5d2.dtb usr
etc linuxrc pd_file sbin var
factory_file lost+found proc sys zImage

- Third try

The last try I have done was to try to mount / as a layer into /.

# mount -t aufs -o br=/mnt/pd=rw:/mnt/update=ro:/mnt/factory=ro:/=ro:/mnt/boot=ro,udba=reval none /

# mount
[...]
none on / type aufs (rw,relatime,si=73ed391d)

Once again the mount command seems to work but files aren't present into /

# ls /
bin etc lib linuxrc mnt proc run sys usr
dev home lib32 media opt root sbin tmp var

How could I achieve my goal of having my different partitions mounted into / as aufs layered root filesystem ?