1 page.title=Traditional 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 <img style="float: right; margin: 0px 15px 15px 15px;" src="images/ape_fwk_hal_extstor.png" alt="Android external storage HAL icon"/> 24 25 <p>Android supports devices with traditional storage, which is defined to be a 26 case-insensitive filesystem with immutable POSIX permission classes and modes. 27 The notion of traditional storage encompasses emulated and portable storage. 28 Portable storage is defined as any external storage that is not <a href="{@docRoot}devices/storage/adoptable.html"> 29 adopted</a> by the 30 system and therefore not formatted and encrypted or tied to a specific device. 31 Because traditional external storage offers minimal protection for stored data, 32 system code should not store sensitive data on external storage. Specifically, 33 configuration and log files should only be stored on internal storage where 34 they can be effectively protected.</p> 35 36 <h2 id="multi-user-external-storage">Multi-user external storage</h2> 37 <p>Starting in Android 4.2, devices can support multiple users, and external 38 storage must meet the following constraints:</p> 39 <ul> 40 <li>Each user must have their own isolated primary external storage, and 41 must not have access to the primary external storage of other users.</li> 42 <li>The <code>/sdcard</code> path must resolve to the correct user-specific 43 primary external storage based on the user a process is running as.</li> 44 <li>Storage for large OBB files in the <code>Android/obb</code> directory 45 may be shared between multiple users as an optimization.</li> 46 <li>Secondary external storage must not be writable by apps, except in 47 package-specific directories as allowed by synthesized permissions.</li> 48 </ul> 49 <p>The default platform implementation of this feature leverages Linux kernel 50 namespaces to create isolated mount tables for each Zygote-forked process, 51 and then uses bind mounts to offer the correct user-specific primary external 52 storage into that private namespace.</p> 53 <p>At boot, the system mounts a single emulated external storage FUSE daemon 54 at <code>EMULATED_STORAGE_SOURCE</code>, which is hidden from apps. After 55 the Zygote forks, it bind mounts the appropriate user-specific subdirectory 56 from under the FUSE daemon to <code>EMULATED_STORAGE_TARGET</code> so that 57 external storage paths resolve correctly for the app. Because an app lacks 58 accessible mount points for other users' storage, they can only access 59 storage for the user it was started as.</p> 60 <p>This implementation also uses the shared subtree kernel feature to 61 propagate mount events from the default root namespace into app namespaces, 62 which ensures that features like ASEC containers and OBB mounting continue 63 working correctly. It does this by mounting the rootfs as shared, and then 64 remounting it as slave after each Zygote namespace is created.</p> 65 66 <h2 id="multiple-external-storage-devices">Multiple external storage devices</h2> 67 <p>Starting in Android 4.4, multiple external storage devices are surfaced 68 to developers through <code>Context.getExternalFilesDirs()</code>, 69 <code>Context.getExternalCacheDirs()</code>, and 70 <code>Context.getObbDirs()</code>.</p> 71 </p>External storage devices surfaced through these APIs must be a 72 semi-permanent part of the device (such as an SD card slot in a battery 73 compartment). Developers expect data stored in these locations to be 74 available over long periods of time. For this reason, transient storage 75 devices (such as USB mass storage drives) should not be surfaced through 76 these APIs.</p> 77 <p>The <code>WRITE_EXTERNAL_STORAGE</code> permission must only grant write 78 access to the primary external storage on a device. Apps must not be 79 allowed to write to secondary external storage devices, except in their 80 package-specific directories as allowed by synthesized 81 permissions. Restricting writes in this way ensures the system can clean 82 up files when applications are uninstalled.</p> 83 84 <h2 id=support_usb_media>USB media support</h2> 85 86 <p>Android 6.0 supports portable storage devices which are only connected to the 87 device for a short period of time, like USB flash drives. When a user inserts a 88 new portable device, the platform shows a notification to let them copy or 89 manage the contents of that device.</p> 90 91 <p>In Android 6.0, any device that is not adopted is considered portable. Because 92 portable storage is connected for only a short time, the platform avoids heavy 93 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 94 blocked for privacy and security reasons.</p> 95
