Which specific storage space is low, when getting "Low Space" on Samsung Galaxy S?

I'm getting the alert "Phone storage space is getting low". Here is my Samsung Galaxy S running Cyanogenmod 9's space status:

Name: Cache

Path: /cache
SIZE: 17.50MB
USED: 2.47MB
FREE: 15.02MB


Name: Data
Path: /data
SIZE: 1889.83MB
USED: 576.64MB

FREE: 1313.19MB


Name: SD-card
Path: /sdcard
SIZE: 5654.56MB
USED: 2011.00MB
FREE: 3643.56MB



Name: System
Path: /system
SIZE: 250.00MB
USED: 240.23MB
FREE: 9.76MB

You'd think it's /system, but /system has been like that forever. In fact, after installing cyanogen it only had 1mb free. I actually deleted some system apps (wallpaper crap mostly) to free up a little more.

I am not asking how to get rid of the alert. I want to know which specific storage space does the alert refer to?

Answer

It is usually /datadata that gets full on the Galaxy S. Interestingly this partition is not displayed in the Storage Setting Menu. The quirk is that /datadata is mounted on a special very fast flash chip that only provides 170MiB of storage space.

While this was enough in the "early days" of Android when the Galaxy S was released, it can easly become tight with modern Apps. /datadata is used to store application user data, and while many apps only place a few KiB there, some apps require to store more. Typical candidates that store around 20MiB in /datadata are Google Chrome and Google Currents.

ATM the best practice is a fix that lets you control which apps can store data on the fast chip and which are outsourced to the slower RAM. This script can also be installed with an app called datafix.

To find the apps that consume the most space use su && du -s /datadata/* |sort -gr |head. For example on my device:

shell@android:/datadata # su && du -s /datadata/* |sort -gr |head                    
39180   /datadata/com.android.chrome
20809   /datadata/com.google.android.apps.currents
16763   /datadata/com.google.earth
9431    /datadata/com.google.android.apps.plus
8938    /datadata/com.inisoft.mediaplayer.a
8270    /datadata/com.google.android.apps.maps

3015    /datadata/com.google.android.apps.docs
2160    /datadata/udk.android.reader
2096    /datadata/com.android.providers.contacts
1597    /datadata/com.fsck.k9