If I sum up some key points from this thread... there is no way of writing to system partition (Legend, Desire...) even if it is mounted in read-write mode. The only way of writing to system partition is via recovery mode (if you have already rooted your Legend) which makes a process of updating/testing a pain... you have to reboot your phone into recovery mode each time you want to change something on system partition. So guys at Desire subforum came up with an idea to put a writable overlay over the /system directory. This principle is very well known from live CD distros of Linux and Asus also introduced this on their EEE systems running Linux.
This method can be used not only on system partition (so we can "write" to it) but also on data partition which brings us really a neat way of point-in-time backup. Basically one could make overlay of an entire system just after the system was installed and set-up (with all software and custom settings). Then when anything goes wrong after this point, all one has to do is to wipe the directory/partition where writes were being made - no need to factory reset (wiping data partition), setting things up and installing all the apps all over again. This method can well be a better alternative of nandroid backup, because restoring from nandroid backup wipes out all partitions and restore data to them from images. And we all know that frequent repetition of this process really deteriorates non-removable NAND flash chips in our phones.
- shell script that does the actual /system overlay
- statically compiled mount (for compatibility reasons)
- auplink utility for flushing pseudo-links to writable device (not used atm)
Shell script that does the overlay is made so it can use any (first) ext2/3 partition on your sdcard. That means you can use your a2sd partition for overlay too. If a2sd is not active but you still have ext2/3 partition on your sdcard, it will be mounted and used for overlay. If you don't have ext2/3 partition on your sdcard, then internal data partition will be used instead. In this case you will have to be careful not to fill it up entirely.
- rooted Legend of course
- any kernel with built-in aufs support
- run-parts integrated in init.rc script (/etc/init.d)
- a2sd script should run before the overlay script (only if you use a2sd)
(When I mention a2sd in this post I mean any a2sd; a2sd, a2sd+, apps2sd, DarkTremor, LightSabre, GoldenThrone )
Boot to recovery mode (ClockworkMod) and apply this update. After applying it (re)boot your phone. At this point overlay is not yet enabled. You have to issue:
adb shell aufs --enable
If you want to enable/disable overlay later on, issue:
adb shell aufs --disable
adb shell aufs --enable
Be advised that after wiping (Factory data reset) overlay is disabled so you will have to enable it again.
Make logcat of boot process and check for errors in 1st 50 or so lines.
Please, do not even try to flash this "update" if you don't know what you are doing or what things I mentioned above mean!!!