1 <html devsite> 2 <head> 3 <title>Traditional Storage</title> 4 <meta name="project_path" value="/_project.yaml" /> 5 <meta name="book_path" value="/_book.yaml" /> 6 </head> 7 <body> 8 <!-- 9 Copyright 2017 The Android Open Source Project 10 11 Licensed under the Apache License, Version 2.0 (the "License"); 12 you may not use this file except in compliance with the License. 13 You may obtain a copy of the License at 14 15 http://www.apache.org/licenses/LICENSE-2.0 16 17 Unless required by applicable law or agreed to in writing, software 18 distributed under the License is distributed on an "AS IS" BASIS, 19 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 20 See the License for the specific language governing permissions and 21 limitations under the License. 22 --> 23 24 25 26 <img style="float: right; margin: 0px 15px 15px 15px;" src="images/ape_fwk_hal_extstor.png" alt="Android external storage HAL icon"/> 27 28 <p>Android supports devices with traditional storage, which is defined to be a 29 case-insensitive filesystem with immutable POSIX permission classes and modes. 30 The notion of traditional storage encompasses emulated and portable storage. 31 Portable storage is defined as any external storage that is not <a href="/devices/storage/adoptable.html"> 32 adopted</a> by the 33 system and therefore not formatted and encrypted or tied to a specific device. 34 Because traditional external storage offers minimal protection for stored data, 35 system code should not store sensitive data on external storage. Specifically, 36 configuration and log files should only be stored on internal storage where 37 they can be effectively protected.</p> 38 39 <h2 id="multi-user-external-storage">Multi-user external storage</h2> 40 <p>Starting in Android 4.2, devices can support multiple users, and external 41 storage must meet the following constraints:</p> 42 <ul> 43 <li>Each user must have their own isolated primary external storage, and 44 must not have access to the primary external storage of other users.</li> 45 <li>The <code>/sdcard</code> path must resolve to the correct user-specific 46 primary external storage based on the user a process is running as.</li> 47 <li>Storage for large OBB files in the <code>Android/obb</code> directory 48 may be shared between multiple users as an optimization.</li> 49 <li>Secondary external storage must not be writable by apps, except in 50 package-specific directories as allowed by synthesized permissions.</li> 51 </ul> 52 <p>The default platform implementation of this feature leverages Linux kernel 53 namespaces to create isolated mount tables for each Zygote-forked process, 54 and then uses bind mounts to offer the correct user-specific primary external 55 storage into that private namespace.</p> 56 <p>At boot, the system mounts a single emulated external storage FUSE daemon 57 at <code>EMULATED_STORAGE_SOURCE</code>, which is hidden from apps. After 58 the Zygote forks, it bind mounts the appropriate user-specific subdirectory 59 from under the FUSE daemon to <code>EMULATED_STORAGE_TARGET</code> so that 60 external storage paths resolve correctly for the app. Because an app lacks 61 accessible mount points for other users' storage, they can only access 62 storage for the user it was started as.</p> 63 <p>This implementation also uses the shared subtree kernel feature to 64 propagate mount events from the default root namespace into app namespaces, 65 which ensures that features like ASEC containers and OBB mounting continue 66 working correctly. It does this by mounting the rootfs as shared, and then 67 remounting it as slave after each Zygote namespace is created.</p> 68 69 <h2 id="multiple-external-storage-devices">Multiple external storage devices</h2> 70 <p>Starting in Android 4.4, multiple external storage devices are surfaced 71 to developers through <code>Context.getExternalFilesDirs()</code>, 72 <code>Context.getExternalCacheDirs()</code>, and 73 <code>Context.getObbDirs()</code>.</p> 74 </p>External storage devices surfaced through these APIs must be a 75 semi-permanent part of the device (such as an SD card slot in a battery 76 compartment). Developers expect data stored in these locations to be 77 available over long periods of time. For this reason, transient storage 78 devices (such as USB mass storage drives) should not be surfaced through 79 these APIs.</p> 80 <p>The <code>WRITE_EXTERNAL_STORAGE</code> permission must only grant write 81 access to the primary external storage on a device. Apps must not be 82 allowed to write to secondary external storage devices, except in their 83 package-specific directories as allowed by synthesized 84 permissions. Restricting writes in this way ensures the system can clean 85 up files when applications are uninstalled.</p> 86 87 <h2 id=support_usb_media>USB media support</h2> 88 89 <p>Android 6.0 supports portable storage devices which are only connected to the 90 device for a short period of time, like USB flash drives. When a user inserts a 91 new portable device, the platform shows a notification to let them copy or 92 manage the contents of that device.</p> 93 94 <p>In Android 6.0, any device that is not adopted is considered portable. Because 95 portable storage is connected for only a short time, the platform avoids heavy 96 operations such as media scanning. Third-party apps must go through the <a href="https://developer.android.com/guide/topics/providers/document-provider.html">Storage Access Framework</a> to interact with files on portable storage; direct access is explicitly 97 blocked for privacy and security reasons.</p> 98 99 </body> 100 </html> 101
