src/devices/storage/config.jd

page.title=Device Configuration
@jd:body
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    <h2>In this document</h2>
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<p>External storage is managed by a combination of the <code>vold</code> init
service and <code>MountService</code> system service. 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>

<h2 id=file_mappings>File mappings</h2>
<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>, <code>noemulatedsd</code>, and <code>encryptable=userdata</code>.</li>
</ul>

<h2 id=configuration_details>Configuration details</h2>
<p>External storage interactions at and above the framework level are handled
through <code>MountService</code>. 
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.

<h3 id=android_5_x_and_earlier>Android 5.x and earlier</h3>
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>

<h3 id=android_6_0>Android 6.0</h3>
<p>Configuration of the storage subsystem is now concentrated in the
device-specific <code>fstab</code> file, and several historical static configuration files/variables have been
removed to support more dynamic behavior:</p>
<ul>
   <li>The <code>storage_list.xml</code> resource overlay has been removed and is no longer used by the framework.
Storage devices are now configured dynamically when detected by <code>vold</code>.
   <li>The <code>EMULATED_STORAGE_SOURCE/TARGET</code> 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 <code>vold</code> at runtime.
  <ul>
     <li>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, <code>/storage/ABCD-1234/report.txt</code> is clearly a different file than <code>/storage/DCBA-4321/report.txt</code>.)
  </ul>
   <li>The hard-coded FUSE services have been removed from device-specific <code>init.rc</code> files and are instead forked dynamically from <code>vold</code> when needed.
</ul>
<p>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. </p>

<h4 id=adoptable_storage>Adoptable storage </h4>
<p>To indicate an adoptable storage device in the <code>fstab</code>, use the <code>encryptable=userdata</code> attribute in the <code>fs_mgr_flags</code> field. Here’s a typical definition:</p>
<pre><code>/devices/platform/mtk-msdc.1/mmc_host*           auto      auto     defaults
voldmanaged=sdcard1:auto,encryptable=userdata
</code></pre>
<p>When a storage device is adopted, the platform erases the contents and writes a
GUID partition table that defines two partitions:</p>
<ul>
   <li>a small empty <code>android_meta</code> partition that is reserved for future use. The partition type GUID is
19A710A2-B3CA-11E4-B026-10604B889DCF.
   <li>a large <code>android_ext</code> partition that is encrypted using dm-crypt and formatted using either <code>ext4</code> or <code>f2fs</code> depending on the kernel capabilities. The partition type GUID is
193D1EA4-B3CA-11E4-B075-10604B889DCF.
</ul>
<h4 id=portable_storage>Portable storage </h4>
<p>In the <code>fstab</code>, storage devices with the <code>voldmanaged</code> attribute are considered to be portable by default unless another attribute
like <code>encryptable=userdata</code> is defined. For example, here’s a typical definition for USB OTG devices:</p>
<pre><code>/devices/*/xhci-hcd.0.auto/usb*             auto            auto    defaults
                                                    voldmanaged=usb:auto
</code></pre>
<p>The platform uses <code>blkid</code> to detect filesystem types before mounting, and users can choose to format the
media when the filesystem is unsupported.</p>