External storage is managed by a combination of the vold init service and StorageManagerService 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.

File mappings

For Android 4.2.2 and earlier, 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.

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, noemulatedsd, and encryptable=userdata.

Configuration details

External storage interactions at and above the framework level are handled through MountService. Due to configuration changes in Android 6.0 (like the removal of the storage_list.xml resource overlay), the configuration details are split into two categories.

Android 5.x and earlier

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.

Android 6.0

Configuration of the storage subsystem is now concentrated in the device-specific fstab file, and several historical static configuration files/variables have been removed to support more dynamic behavior:

• The storage_list.xml resource overlay has been removed and is no longer used by the framework. Storage devices are now configured dynamically when detected by vold.
• The EMULATED_STORAGE_SOURCE/TARGET environment variables have been removed and are no longer used by Zygote to configure user-specific mount points. Instead, user separation is now enforced with user-specific GIDs, and primary shared storage is mounted into place by vold at runtime.
  • Developers may continue to build paths dynamically or statically depending on their use case. Including the UUID in the path identifies each card to make location clearer for developers. (For example, /storage/ABCD-1234/report.txt is clearly a different file than /storage/DCBA-4321/report.txt.)
• The hard-coded FUSE services have been removed from device-specific init.rc files and are instead forked dynamically from vold when needed.

In addition to these configuration changes, Android 6.0 includes the notion of adoptable storage. For Android 6.0 devices, any physical media that is not adopted is viewed as portable.

Adoptable storage

To indicate an adoptable storage device in the fstab, use the encryptable=userdata attribute in the fs_mgr_flags field. Here’s a typical definition:

/devices/platform/mtk-msdc.1/mmc_host*           auto      auto     defaults
voldmanaged=sdcard1:auto,encryptable=userdata

When a storage device is adopted, the platform erases the contents and writes a GUID partition table that defines two partitions:

• a small empty android_meta partition that is reserved for future use. The partition type GUID is 19A710A2-B3CA-11E4-B026-10604B889DCF.
• a large android_ext partition that is encrypted using dm-crypt and formatted using either ext4 or f2fs depending on the kernel capabilities. The partition type GUID is 193D1EA4-B3CA-11E4-B075-10604B889DCF.

Portable storage

In the fstab, storage devices with the voldmanaged attribute are considered to be portable by default unless another attribute like encryptable=userdata is defined. For example, here’s a typical definition for USB OTG devices:

/devices/*/xhci-hcd.0.auto/usb*             auto            auto    defaults
                                                    voldmanaged=usb:auto

The platform uses blkid to detect filesystem types before mounting, and users can choose to format the media when the filesystem is unsupported.