1 page.title=Adoptable Storage 2 @jd:body 3 <!-- 4 Copyright 2015 The Android Open Source Project 5 Licensed under the Apache License, Version 2.0 (the "License"); 6 you may not use this file except in compliance with the License. 7 You may obtain a copy of the License at 8 http://www.apache.org/licenses/LICENSE-2.0 9 Unless required by applicable law or agreed to in writing, software 10 distributed under the License is distributed on an "AS IS" BASIS, 11 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 See the License for the specific language governing permissions and 13 limitations under the License. 14 --> 15 <div id="qv-wrapper"> 16 <div id="qv"> 17 <h2>In this document</h2> 18 <ol id="auto-toc"> 19 </ol> 20 </div> 21 </div> 22 23 24 <p>Android has always supported external storage accessories (such as SD cards), but 25 these accessories were historically limited to simple file storage, due to 26 their expected impermanence and the minimal data protection offered to 27 <a href="{@docRoot}devices/storage/traditional.html">traditional external storage</a>. 28 Android 6.0 introduces the ability to 29 <a href="https://developer.android.com/preview/behavior-changes.html#behavior-adoptable-storage">adopt</a> 30 external storage media to act like internal storage.</p> 31 32 <p>When external storage media is adopted, its formatted and encrypted to only 33 work with a single Android device at a time. Because the media is strongly tied 34 to the Android device that adopted it, it can safely store both apps and 35 private data for all users.</p> 36 37 <p>When users insert new storage media (such as an SD card) in an adoptable 38 location, Android asks them how they want to use the media. They can choose to 39 adopt the media, which formats and encrypts it, or they can continue using it 40 as-is for simple file storage. If they choose to adopt, the platform offers to 41 migrate the primary shared storage contents (typically mounted at <code>/sdcard</code>) 42 to the newly adopted media, freeing up valuable space on internal storage.</p> 43 44 <p>Apps can be placed on adopted storage media only when the developer has 45 indicated support through the <code>android:installLocation</code> attribute. 46 New installs of supported apps are automatically placed on the 47 storage device with the most free space, and users can move supported apps 48 between storage devices in the <em>Settings</em> app. Apps moved to adopted 49 media are remembered while the media is ejected, 50 and return when the media is reinserted.</p> 51 52 <h2 id=security>Security</h2> 53 54 55 <p>The platform randomly generates an encryption key for each adopted device, 56 and that key is stored on the internal storage of the Android device. This 57 effectively makes the adopted media as secure as internal storage. Keys are 58 associated with adopted devices based on the adopted partition GUID. The 59 adopted device is encrypted using <code>dm-crypt</code> configured with the 60 <code>aes-cbc-essiv:sha256</code> algorithm and a 128-bit key size.</p> 61 62 <p>The on-disk layout of the adopted device closely mirrors the internal data 63 partition, including SELinux labels, etc. When multi-user is supported on the 64 Android device, the adopted storage device also supports multi-user with the 65 same level of isolation as internal storage.</p> 66 67 <p>Because the contents of an adopted storage device are strongly tied to the 68 Android device that adopted it, the encryption keys should not be extractable 69 from the parent device, and therefore the storage device can't be mounted elsewhere.</p> 70 71 <h2 id=performance_and_stability>Performance and stability</h2> 72 73 74 <p>Only external storage media in stable locations, such as a slot inside a 75 battery compartment or behind a protective cover, should be considered for 76 adoption to help avoid accidental data loss or corruption. In particular, USB 77 devices connected to a phone or tablet should never be considered for adoption. 78 One common exception would be an external USB drive connected to a TV-style 79 device, because the entire TV is typically installed in a stable location.</p> 80 81 <p>When a user adopts a new storage device, the platform runs a benchmark and 82 compares its performance against internal storage. If the adopted device is 83 significantly slower than internal storage, the platform warns the user about a 84 possibly degraded experience. This benchmark was derived from the actual I/O 85 behavior of popular Android apps. Currently, the AOSP implementation will only 86 warn users beyond a single threshold, but device manufacturers may adapt this 87 further, such as rejecting adoption completely if the card is extremely slow.</p> 88 89 <p>Adopted devices must be formatted with a filesystem that supports POSIX 90 permissions and extended attributes, such as <code>ext4</code> or <code>f2fs</code>. 91 For optimal performance, the <code>f2fs</code> filesystem is recommended for 92 flash-based storage devices.</p> 93 94 <p>When performing periodic idle maintenance, the platform issues <code>FI_TRIM</code> 95 to adopted media just like it does for internal storage. The current SD card 96 specification does not support the <code>DISCARD</code> command; but the kernel 97 instead falls back to the <code>ERASE</code> command, which SD card firmware 98 may choose to use for optimization purposes.</p> 99
