Why do I get "Phone storage low" warnings on my Android device?

On an Android device, there is internal storage and external storage.

Internal storage is non-volatile NAND memory built into the device itself. It is also referred to as "ROM" on some spec sheets. Think of it as the "C:" drive on the PC where the Windows OS is located, and all other applications are installed. As such, not all internal storage is freely available to the user. After accounting for the system files (i.e. Windows OS), only the remainder can be used by applications. This free space is known as "Application storage" or "Phone storage". For example, the Google Nexus One has 512MB internal storage, but only 190MB phone storage.

External storage is the microSD card that you plug into the device. This can be as large as 32GB on most devices.

The "Phone storage low" warning occurs because earlier Android devices typically come with very low internal memory (192MB~512MB). Every application (and the data it creates) goes into phone storage and very soon it gets filled up, even though the external storage is practically untouched.

From Android 2.2 (Froyo), an application can write to a new API (A2SD) to let the OS move parts of it to external storage. However only parts of the application is moved, leaving substantial portions of it (libraries, data, cache) in phone storage. So over time, phone storage still gets clogged up.

The Apple iPhone decidedly does not have this problem because it comes with large internal storage (and does not support external memory cards anyway). With the latest Android devices (eg. Nexus S), Google seems to be moving in this direction as well. With 8 or 16GB of internal storage, phone storage low warnings should be a thing of the past.

For older devices, there are a few workarounds which I will explore in later posts. They range from the simplest (running a simple command) to the most difficult (flashing a custom ROM). Alas, from a user's point of view, these workarounds do not help achieve the holy grail, which is fully automatic and transparent storage management. But they will have to do for those of us struggling with low phone storage.