src/devices/storage/config.jd

page.title=Device Specific Configuration
@jd:body

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<p>External storage is managed by a combination of the <code>vold</code> init
service and <code>MountService</code> system servic. Mounting of physical
external storage volumes is handled by <code>vold</code>, which performs
staging operations to prepare the media before exposing it to apps.</p>

<p>For Android 4.2.2 and earlier, the device-specific <code>vold.fstab</code>
configuration file defines mappings from sysfs devices to filesystem mount
points, and each line follows this format:</p>

<pre><code>dev_mount &lt;label&gt; &lt;mount_point&gt; &lt;partition&gt; &lt;sysfs_path&gt; [flags]
</code></pre>

<ul>
<li><code>label</code>: Label for the volume.</li>
<li><code>mount_point</code>: Filesystem path where the volume should be mounted.</li>
<li><code>partition</code>: Partition number (1 based), or 'auto' for first usable partition.</li>
<li><code>sysfs_path</code>: One or more sysfs paths to devices that can provide this mount
point. Separated by spaces, and each must start with <code>/</code>.</li>
<li><code>flags</code>: Optional comma separated list of flags, must not contain <code>/</code>.
Possible values include <code>nonremovable</code> and <code>encryptable</code>.</li>
</ul>

<p>For Android releases 4.3 and later, the various fstab files used by init, vold and
recovery were unified in the <code>/fstab.&lt;device&gt;</code> file. For external
storage volumes that are managed by <code>vold</code>, the entries should have the
following format:</p>
<pre><code>&lt;src&gt; &lt;mnt_point&gt; &lt;type&gt; &lt;mnt_flags&gt; &lt;fs_mgr_flags&gt;
</code></pre>
<ul>
<li><code>src</code>: A path under sysfs (usually mounted at /sys) to the device that
can provide the mount point. The path must start with <code>/</code>.</li>
<li><code>mount_point</code>: Filesystem path where the volume should be mounted.</li>
<li><code>type</code>: The type of the filesystem on the volume. For external cards,
this is usually <code>vfat</code>.</li>
<li><code>mnt_flags</code>: <code>Vold</code> ignores this field and it should be set
to <code>defaults</code></li>
<li><code>fs_mgr_flags</code>: <code>Vold</code> ignores any lines in the unified fstab
that do not include the <code>voldmanaged=</code> flag in this field. This flag must
be followed by a label describing the card, and a partition number or the word
<code>auto</code>. Here is an example: <code>voldmanaged=sdcard:auto</code>.
Other possible flags are <code>nonremovable</code>,
<code>encryptable=sdcard</code>, and <code>noemulatedsd</code>.</li>
</ul>
<p>External storage interactions at and above the framework level are handled
through <code>MountService</code>. The device-specific <code>storage_list.xml</code> configuration
file, typically provided through a <code>frameworks/base</code> overlay, defines the
attributes and constraints of storage devices. The <code>&lt;StorageList&gt;</code> element
contains one or more <code>&lt;storage&gt;</code> elements, exactly one of which should be marked
primary. <code>&lt;storage&gt;</code> attributes include:</p>
<ul>
<li><code>mountPoint</code>: filesystem path of this mount.</li>
<li><code>storageDescription</code>: string resource that describes this mount.</li>
<li><code>primary</code>: true if this mount is the primary external storage.</li>
<li><code>removable</code>: true if this mount has removable media, such as a physical SD
card.</li>
<li><code>emulated</code>: true if this mount is emulated and is backed by internal storage,
possibly using a FUSE daemon.</li>
<li><code>mtp-reserve</code>: number of MB of storage that MTP should reserve for free
storage. Only used when mount is marked as emulated.</li>
<li><code>allowMassStorage</code>: true if this mount can be shared via USB mass storage.</li>
<li><code>maxFileSize</code>: maximum file size in MB.</li>
</ul>
<p>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 <code>system/core/sdcard</code>, which can
be added as a device-specific <code>init.rc</code> service:</p>
<pre><code># virtual sdcard daemon running as media_rw (1023)
service sdcard /system/bin/sdcard &lt;source_path&gt; &lt;dest_path&gt; 1023 1023
    class late_start
</code></pre>
<p>Where <code>source_path</code> is the backing internal storage and <code>dest_path</code> is the
target mount point.</p>
<p>When configuring a device-specific <code>init.rc</code> script, the <code>EXTERNAL_STORAGE</code>
environment variable must be defined as the path to the primary external
storage. The <code>/sdcard</code> 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.</p>