In Android 8.0, the MountService class was renamed to StorageManagerService.

Device Configuration

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.

Note: In Android 8.0, the 

MountService

 class was renamed to 

StorageManagerService

.

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 

StorageManagerService

.
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.