Android supports devices with external storage, which is defined to be a case-insensitive filesystem with immutable POSIX permission classes and modes. External storage can be provided by physical media (such as an SD card), or by exposing a portion of internal storage through an emulation layer(外边存储器可以由物理介质像SD卡或者通过内部存储器的部分空间模拟来提供). Devices may contain multiple instances of 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. Starting in Android 4.1, read access is protected with the READ_EXTERNAL_STORAGE permission.(要想获得对整个外部存储器的访问权限采用的方法)


Starting in Android 4.4, the owner, group and modes of files on external storage devices are now synthesized based on directory structure. This enables apps to manage their package-specific directories on external storage without requiring they hold the broad WRITE_EXTERNAL_STORAGE permission. For example, the app with package name can now freely access Android/data/ on external storage devices with no permissions. These synthesized permissions are accomplished by wrapping raw storage devices in a FUSE daemon.(从Android4.4开始,得益于FUSE守护进程的包装,app可以随意访问外边存储器上自己包特定的目录,不需要再申请访问权限)


Since external storage offers minimal protection for stored data, 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.(由于外边存储器提供对存储的数据的最小的保护,因此,重要的数据如配置或log应该保存在内部存储器上)


Multiple external storage devices

Starting in Android 4.4, multiple external storage devices are surfaced to developers throughContext.getExternalFilesDirs(), Context.getExternalCacheDirs(), and Context.getObbDirs().

External storage devices surfaced through these APIs must be a semi-permanent part of the device (such as an SD card slot in a battery compartment). Developers expect data stored in these locations to be available over long periods of time. For this reason, transient storage devices (such as USB mass storage drives) should not be surfaced through these APIs.

The WRITE_EXTERNAL_STORAGE permission must only grant write access to the primary external storage on a device. Apps must not be allowed to write to secondary external storage devices, except in their package-specific directories as allowed by synthesized permissions. Restricting writes in this way ensures the system can clean up files when applications are uninstalled.


Multi-user external storage

Starting in Android 4.2, devices can support multiple users, and external storage must meet the following constraints:

  • Each user must have their own isolated primary external storage, and must not have access to the primary external storage of other users.
  • The /sdcard path must resolve to the correct user-specific primary external storage based on the user a process is running as.
  • Storage for large OBB files in the Android/obb directory may be shared between multiple users as an optimization.
  • Secondary external storage must not be writable by apps, except in package-specific directories as allowed by synthesized permissions.

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.


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.


For Android 4.2.2 and earlier, the device-specific vold.fstab configuration file defines mappings from sysfs devices(设备节点对应的文件) 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 andencryptable.


For Android releases 4.3 and later, the various fstab files used by init, vold and recovery were unified in the/fstab.<device> file. For external storage volumes that are managed by vold, the entries should have the following format:

<src> <mnt_point> <type> <mnt_flags> <fs_mgr_flags>
  • src: A path under sysfs (usually mounted at /sys) to the device that can provide the mount point. The path must start with /.
  • mount_point: Filesystem path where the volume should be mounted.
  • type: The type of the filesystem on the volume. For external cards, this is usually vfat.
  • mnt_flags: Vold ignores this field and it should be set to defaults
  • fs_mgr_flags: Vold ignores any lines in the unified fstab that do not include the voldmanaged= flag in this field. This flag must be followed by a label describing the card, and a partition number or the word auto. Here is an example: voldmanaged=sdcard:auto. Other possible flags are nonremovable, encryptable=sdcard, andnoemulatedsd.


External storage interactions at and above the framework level are handled through MountService. (从framework层开始向上与外边设备的交互由MountService负责)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:(下面是sdcard service描述,它可以用内部设备来模拟外部设备)

# 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.



posted @ 2013-11-02 14:31  sky-zhang  阅读(4098)  评论(0编辑  收藏  举报