Working with Emulator Disk Images

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The emulator uses mountable disk images stored on your development machine to simulate flash (or similar) partitions on an actual device. For example, it uses disk image containing an emulator-specific kernel, the Android system, a ramdisk image, and writeable images for user data and simulated SD card.

To run properly, the emulator requires access to a specific set of disk image files. By default, the Emulator always looks for the disk images in the private storage area of the AVD in use. If no images exist there when the Emulator is launched, it creates the images in the AVD directory based on default versions stored in the SDK.

Note: The default storage location for AVDs is in ~/.android/avd on OS X and Linux, C:\Documents and Settings\<user>\.android\ on Windows XP, and C:\Users\<user>\.android\ on Windows Vista.

To let you use alternate or custom versions of the image files, the emulator provides startup options that override the default locations and filenames of the image files. When you use the options, the emulator searches for the image file under the image name or location that you specify; if it can not locate the image, it reverts to using the default names and location.

The emulator uses three types of image files: default image files, runtime image files, and temporary image files. The sections below describe how to override the location/name of each type of file.

Default Images

When the emulator launches but does not find an existing user data image in the active AVD's storage area, it creates a new one from a default version included in the SDK. The default user data image is read-only. The image files are read-only.

The emulator provides the -system <dir> startup option to let you override the location under which the emulator looks for the default user data image.

The emulator also provides a startup option that lets you override the name of the default user data image, as described in the table below. When you use the option, the emulator looks in the default directory, or in a custom location (if you specified -system <dir>).

Name

Description

Comments

userdata.img

The initial user-data disk image

Override using -initdata <file>. Also see -data <file>, below.

Runtime Images: User Data and SD Card

At runtime, the emulator reads and writes data on two disk images: a user-data image and (optionally) an SD card image. This emulates the user-data partition and removable storage media on actual device.

The emulator provides a default user-data disk image. At startup, the emulator creates the default image as a copy of the system user-data image (user-data.img), described above. The emulator stores the new image with the files of the active AVD.

The emulator provides startup options to let you override the actual names and storage locations of the runtime images to load, as described in the table below. When you use one of these options, the emulator looks for the specified file(s) in the current working directory, in the AVD directory, or in a custom location (if you specified a path with the filename).

Name

Description

Comments

userdata-qemu.img

An image to which the emulator writes runtime user-data for a unique user.

Override using -data <filepath>, where <filepath> is the path the image, relative to the current working directory. If you supply a filename only, the emulator looks for the file in the current working directory. If the file at <filepath> does not exist, the emulator creates an image from the default userdata.img, stores it under the name you specified, and persists user data to it at shutdown.

sdcard.img

An image representing an SD card inserted into the emulated device.

Override using -sdcard <filepath>, where <filepath> is the path the image, relative to the current working directory. If you supply a filename only, the emulator looks for the file in the current working directory.

User-Data Image

Each emulator instance uses a writeable user-data image to store user- and session-specific data. For example, it uses the image to store a unique user's installed application data, settings, databases, and files.

At startup, the emulator attempts to load a user-data image stored during a previous session. It looks for the file in the current working directory, in the AVD directory as described above, and at the custom location/name that you specified at startup.

• If it finds a user-data image, it mounts the image and makes it available to the system for reading/writing of user data.

• If it does not find one, it creates an image by copying the system user-data image (userdata.img), described above. At device power-off, the system persists the user data to the image, so that it will be available in the next session. Note that the emulator stores the new disk image at the location/name that you specify in -data startup option.

Note: Because of the AVD configurations used in the emulator, each emulator instance now gets its own dedicated storage. There is no need to use the -d option to specify an instance-specific storage area.

SD Card

Optionally, you can create a writeable disk image that the emulator can use to simulate removeable storage in an actual device. For information about how to create an emulated SD card and load it in the emulator, see SD Card Emulation

You can also use the android tool to automatically create an SD Card image for you, when creating an AVD. For more information, see Command-line options for AVDs.

Temporary Images

The emulator creates two writeable images at startup that it deletes at device power-off. The images are:

• A writable copy of the Android system image

• The /cache partition image

The emulator does not permit renaming the temporary system image or persisting it at device power-off.

The /cache partition image is initially empty, and is used by the browser to cache downloaded web pages and images. The emulator provides an -cache <file>, which specifies the name of the file at which to persist the /cache image at device power-off. If <file> does not exist, the emulator creates it as an empty file.

You can also disable the use of the cache partition by specifying the -nocache option at startup.

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    Where is the emulator system image mounted?
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