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9.7. Overlay File Systems

Introduction

Overlay File Systems provide an interesting approach to several frequent problems in Embedded Systems. For example, mini_fo is a virtual kernel file system that can make read-only file systems writable. This is done by redirecting modifying operations to a writeable location called "storage directory", and leaving the original data in the "base directory" untouched. When reading, the file system merges the modifed and original data so that only the newest versions will appear. This occurs transparently to the user, who can access the data like on any other read-write file system.

What it is good for?

In embedded systems the main use of mini_fo is to overlay the root file system. This means it is mounted on top of the regular root file system, thereby allowing applications or users to transparently make modifications to it but redirecting these to a different location.

Some examples of why this is usefull are explained in the following sections.

Making a read-only root filesystem writeable

Root file systems stored in flash are often read only, such as cramfs or read only ext2. While this offers major advantages in terms of speed and flash memory footprint, it nevertheless is often desireable to be able to modify the root file system, for example to

This can be achieved by mounting mini_fo on top of the root file system and using a (probably small) writeable partition as the storage file system. This could be either a JFFS2 flash file system, or during development even an external hard disk. This has the following advantages:

Non persistant changes

Ramdisks are often used when the root file system needs to be modified non-persistantly. This works well, but downsides are the large RAM memory footprint and the time costly operation of copying the ramdisk into RAM during startup. These can be avoided by overlaying the root file system as in the previous example but with the difference that the tmpfs file system is used as storage. Thus only modified files are stored in RAM, and can even be swapped out if neccessary. This saves boot time and RAM!

Resetable changes

Mini_fo can be easily used to implement a "reset to factory defaults" function by overlaying the default root file system. When configuration changes are made, these are automatically directed to the storage file system and take precedence over the original files. Now, to restore the system to factory defaults, all that needs to be done is delete the contents of the storage directory. This will remove all changes made to the root file system and return it to the original state.

ALERT! Note: Deleting the contents of the storage directory should only be done when the overlay file system is unmounted.

Examples

Generally, there are two different ways of overlaying the root file system, which both make sense in different scenarios.

Starting a single application in a chrooted overlayed environment

This is easy. Let's assume "/" is the read-only root file system and /dev/mtdblock5 contains a small JFFS2 flash partition that shall be used to store modifications made by application "/usr/bin/autoPilot":

# mount -t jffs2 /dev/mtdblock5 /tmp/sto
# insmod mini_fo.o
# mount -t mini_fo -o base=/,sto=/tmp/sto/ / /mnt/mini_fo/
# cd /mnt/mini_fo/
# chroot . /usr/bin/autoPilot
The mini_fo file system is mounted with "/" as base directory, "/tmp/sto/" as storage directory to the mount point "/mnt/mini_fo". After that, chroot(1) is used to start the application with the new file system root "/mnt/mini_fo". All modifications made by the application will be stored to the JFFS2 file system in /tmp/sto.

Starting the whole system system in chrooted overlayed environment

This is more interesting, and a bit trickier, as mounting needs to be done during system startup after the root file system has been mounted, but before init is started. The best way to do this is to have a script that mounts the mini_fo file system on top of root and then starts init in the chrooted overlayed environment. For example assume the following script "overlay_init", stored in /sbin/:

#!/bin/bash
#
# mount mini_fo overlay file system and execute init
#

# make sure these exist in the read-only file system
STORAGE=/tmp/sto
MOUNT_POINT=/mnt/mini_fo/

# mount tmpfs as storage file system with a maximum size of 32MB
mount -t tmpfs -o rw,size=32M none $STORAGE

/sbin/modprobe mini_fo
mount -t mini_fo -o base=/,sto=$STORAGE / $MOUNT_POINT

exec /usr/sbin/chroot $MOUNT_POINT /sbin/init

echo "exec chroot failed, bad!"
exec /bin/sh

exit 1

Now its easy to choose between a mini_fo overlayed and the regular non overlayed system just by setting the "init" kernel parameter in the boot loader to "init=/sbin/overlay_init".

Tips

Performance overhead

The mini_fo file system is inserted as an additional layer between the VFS and the native file system, and thus creates some overhead that varies strongly depending of the operation performed.

  1. modifying a regular file for the first time
    This results in a copy of the original file beeing created in the storage directory, that is then modified. Overhead depends on the size of the modified file.
  2. Reading from files, creating new files, modifying already modified files
    These operations are passed directly through to the lower native layer, and only impose an overhead of 1-2%.

Further information

This section discusses how the mini_fo overlay file system can be used in embedded systems. More general information is available at the mini_fo project page: http://www.denx.de/wiki/Know/MiniFOHome.

9.6. Root File System Selection 1. Abstract 9.8. The Persistent RAM File system (PRAMFS)
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