Android ramdisk.img system.img userdata.img

ramdisk.img is a small partition image that is mounted read-only by the 

kernel at boot time. It only contains /init and a few config files. It is 

used to start init which will mount the rest of the system images properly 

and run the init procedure. A Ramdisk is a standard Linux feature.

system.img is a partition image that will be mounted as / and thus contains 

all system binaries 

userdata.img is a partition image that can be mounted as /data and thus 

contains all application-specific and user-specific data.

The build system generates these files, which can later be flashed to a real 

device, however the emulator uses them in a different way:

   - system.img is copied into a temporary file, which is used by the 

   emulator session. So any change you make to / as root in the emulator are 

   lost when the program exits

   - userdata.img is only used when you use -wipe-data. Instead, it uses 

   ~/.android/userdata-qemu.img (on Unix) as the persistent /data partition 

   image. Using -wipe-data simply copes the content of userdata.img into 

   userdata-qemu.img

The main idea being that the emulator should not modify system.img and 

userdata.img since they were generated as device images. However whether a 

given system.img/userdata.img set of images will run on a real device 

properly depends on how it was generated. For example I doubt that the ones 

that come with the SDK would.

ramdisk.img is gziped cpio archive. ramdisk.img is a small partition image that is mounted read-only by the kernel at boot time. It 

 

only contains /init and a few config files. It is used to start init which will mount the rest of the system images properly and run 

 

the init procedure. A Ramdisk is a standard Linux feature. It is made just for the Android and do special things to start up the 

 

Android system. 

system.img is a partition image that will be mounted as / and thus contains all system binaries. 

userdata.img is a partition image that can be mounted as /data and thus contains all application-specific and user-specific data