Android supports devices with external storage, which is defined to be a case-insensitive and permissionless filesystem. External storage can be provided by physical media (such as an SD card), or by an emulation layer backed by internal storage. Devices may contain multiple instances of external storage, but currently only the primary external storage is exposed to developers through API.
External storage is managed by a combination of the vold init service and MountService system service.
Mounting of physical external storage volumes is handled by vold, which performs staging operations to prepare the media before exposing it to apps. The device-specific vold.fstab configuration file defines mappings from sysfs devices to filesystem mount points, and each line follows this format:
dev_mount <label> <mount_point> <partition> <sysfs_path> [flags]
label: Label for the volume.mount_point: Filesystem path where the volume should be mounted.partition: Partition number (1 based), or ‘auto’ for first usable partition.sysfs_path: One or more sysfs paths to devices that can provide this mount point. Separated by spaces, and each must start with /.flags: Optional comma separated list of flags, must not contain /. Possible values include nonremovable and encryptable.External storage interactions at and above the framework level are handled through MountService. The device-specific storage_list.xml configuration file, typically provided through a frameworks/base overlay, defines the attributes and constraints of storage devices. The <StorageList> element contains one or more <storage> elements, exactly one of which should be marked primary. <storage> attributes include:
mountPoint: filesystem path of this mount.storageDescription: string resource that describes this mount.primary: true if this mount is the primary external storage.removable: true if this mount has removable media, such as a physical SD card.emulated: true if this mount is emulated and is backed by internal storage, possibly using a FUSE daemon.mtp-reserve: number of MB of storage that MTP should reserve for free storage. Only used when mount is marked as emulated.allowMassStorage: true if this mount can be shared via USB mass storage.maxFileSize: maximum file size in MB.Devices may provide external storage by emulating a case-insensitive, permissionless filesystem backed by internal storage. One possible implementation is provided by the FUSE daemon in system/core/sdcard, which can be added as a device-specific init.rc service:
# virtual sdcard daemon running as media_rw (1023)
service sdcard /system/bin/sdcard <source_path> <dest_path> 1023 1023
class late_start
Where source_path is the backing internal storage and dest_path is the target mount point.
When configuring a device-specific init.rc script, the EXTERNAL_STORAGE environment variable must be defined as the path to the primary external storage. The /sdcard path must also resolve to the same location, possibly through a symlink. If a device adjusts the location of external storage between platform updates, symlinks should be created so that old paths continue working.
As an example, here’s the storage configuration for Xoom, which uses a FUSE daemon to provide primary external storage, and includes a physical SD card as secondary external storage:
Access to external storage is protected by various Android permissions. Starting in Android 1.0, write access is protected with the WRITE_EXTERNAL_STORAGE permission, implemented using the sdcard_rw GID. Starting in Android 4.1, read access is protected with the new READ_EXTERNAL_STORAGE permission, implemented using the sdcard_r GID. To implement the read permission, a new top-level /storage directory was created such that processes must hold the sdcard_r GID to traverse into it.
Since external storage offers no support for traditional POSIX filesystem permissions, system code should not store sensitive data on external storage. Specifically, configuration and log files should only be stored on internal storage where they can be effectively protected.
Starting in Android 4.2, devices can support multiple users, and external storage must meet the following constraints:
/sdcard path must resolve to the correct user-specific primary external storage based on the user a process is running as.Android/obb directory may be shared between multiple users as an optimization.The default platform implementation of this feature leverages Linux kernel namespaces to create isolated mount tables for each Zygote-forked process, and then uses bind mounts to offer the correct user-specific primary external storage into that private namespace.
At boot, the system mounts a single emulated external storage FUSE daemon at EMULATED_STORAGE_SOURCE, which is hidden from apps. After the Zygote forks, it bind mounts the appropriate user-specific subdirectory from under the FUSE daemon to EMULATED_STORAGE_TARGET so that external storage paths resolve correctly for the app. Because an app lacks accessible mount points for other users’ storage, they can only access storage for the user it was started as.
This implementation also uses the shared subtree kernel feature to propagate mount events from the default root namespace into app namespaces, which ensures that features like ASEC containers and OBB mounting continue working correctly. It does this by mounting the rootfs as shared, and then remounting it as slave after each Zygote namespace is created.