src/devices/tech/storage/index.jd

page.title=External Storage Technical Information
@jd:body

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<p>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. Devices may contain multiple instances of external storage.</p>

<p>Access to external storage is protected by various Android
permissions. Starting in Android 1.0, write access is protected with the
<code>WRITE_EXTERNAL_STORAGE</code> permission. Starting in Android 4.1,
read access is protected with the <code>READ_EXTERNAL_STORAGE</code>
permission.</p>

<p>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
<code>WRITE_EXTERNAL_STORAGE</code> permission. For example, the app with
package name <code>com.example.foo</code> can now freely access
<code>Android/data/com.example.foo/</code> on external storage devices with
no permissions. These synthesized permissions are accomplished by wrapping
raw storage devices in a FUSE daemon.</p>

<p>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.</p>


<h2 id="multiple-external-storage-devices">Multiple external storage devices</h2>

<p>Starting in Android 4.4, multiple external storage devices are surfaced
to developers through <code>Context.getExternalFilesDirs()</code>,
<code>Context.getExternalCacheDirs()</code>, and
<code>Context.getObbDirs()</code>.</p>

</p>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.</p>

<p>The <code>WRITE_EXTERNAL_STORAGE</code> 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.</p>


<h2 id="multi-user-external-storage">Multi-user external storage</h2>

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

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

<p>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.</p>

<p>At boot, the system mounts a single emulated external storage FUSE daemon
at <code>EMULATED_STORAGE_SOURCE</code>, which is hidden from apps. After
the Zygote forks, it bind mounts the appropriate user-specific subdirectory
from under the FUSE daemon to <code>EMULATED_STORAGE_TARGET</code> 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.</p>

<p>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.</p>