Android的镜像文件（image）

1、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.

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

3、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.