How to move dalvik-cache to /system partition in Android

Disclaimer: I'm not responsible; it's your device, your hack, your mod.

This little how-to is for linux.

There's no guarantee it'll work.

If you ever wondered where is you're free space going on your android device - or why do every single app takes up twice the space of the downloaded data - the answer is: dalvik-cache.

Dalvik is a Java Virtual Machine implementation, used in all Android devices. I've got a HTC Desire ( codenamed Bravo if that's better ). In this machine, the user have access to 160 MB - this includes the downloaded apps, the app data and the dalvik cache. No magic here, it's going to be full in a blink.

One way is moving the apps to the SD card, a lot of scripts exists for this already, but this had been really unstable for me - meaning a sudden power loss ment full-reinstall, including the ROM itself.

I searched for an other way: the /system partition is twice the size of the /data ( the userspace ), 250 MB, and it's more than half empty! Why is dalvik-cache not located here?

It's simple: security. Nowadays every manufacturer believes that officially blocking users to take full control of their devices is the way of a good device. No, it is not.

So... what to do? In a few words: make system writeable permanently, delete the /data/dalvik-cache folder, create /system/dalvik-cache folder and make a symlink from this to /data/dalvik-cache.

How? First: root your phone, make it S-OFF, and install your custom ROM. Then....

unpack your custom ROM's zip.
inside, you'll find a boot.img
download these tools, extract them¹
copy boot.img into the extraction folder
run ./extractboot as root
go into the out/ramdisk folder
modify init.rc (see below)
re-pack the boot.img with ./packboot
upload with fastboot fastboot flash boot boot.img
pray

To hack the init.rc file: I'm using "stock" CyanogenMod 7.2.0-rc1, so the following is especially for this. Search for a line where the /system is re-mounted as read only something like this:

mount yaffs2 mtd@system /system ro remount

comment this line out OR replace ro with rw.

Search for the part creating the dalvik-cache, something like this:

# create dalvik-cache and double-check the perms
mkdir /data/dalvik-cache 0771 system system
chown system system /data/dalvik-cache
chmod 0771 /data/dalvik-cache

Modify it:

# create dalvik-cache and double-check the perms
mkdir /system/dalvik-cache 0771 system system
chown system system /system/dalvik-cache
chmod 0771 /system/dalvik-cache

ln -s /system/dalvik-cache /data/dalvik-cache
chown system system /data/dalvik-cache
chmod 0771 /data/dalvik-cache

That' all. Please feel free to ask any questions.

(Oh, by the way: this entry was written by Peter Molnar, and originally posted on petermolnar dot net.)