Vold(Volume Daemon) 主要负责管理和控制外部存储设备，这些控制包括SD卡设备插拔，挂载，格式化等等. Vold在Android 平台上的作用相当于udev在Linux平台上的功能。

Vold的架构

Vold的架构大致如图：

• Kernel：detect device hot-plug, load drivers
• Sysfs: generate events and send it to user space
• NetlinkManager: establish connection to sysfs, listen to uevents and process it
• VolumeManager: disk volume operation
• CommandListener: wait for commands from MountService
• NativeDaemonConnector:establish connection between MountService and vold daemon

其中，一张SD卡对应一个DirectVolume对象。

Vold模块的类图如下：

Vold startup

vold的启动分为两个过程，一是vold daemon的启动，二是Java层MountService的启动。

启动Vold daemon

Vold daemon由init进程启动，参见init.rc片段：

vold入口函数位于/system/vold/mail.cpp，启动后处理流程如下：

1.Create directory /dev/block/vold 1.Create singleton instance of VolumeManager , NetlinkManager andCommandListener 1.Process configuration file /system/etc/vold.fstab 1.NetlinkManager creates connection between kernel and vold 1.Manually trigger uevents by writing “add\n” to file in /sys/block 1.CommandListener starts listening commands from framework

启动MountService

1.MountService initialized by SystemServer on device starts up 1.Create connection to vold

After MountService is up, the vold system is ready.

Send commands from framework

以App层通过MountService 挂载一个volume为例，说明从上层向下层的调用流程。

1. NativeDaemonConnector::sendCommand()
2. SocketListener::runListener() <—vold accepting connections here
3. FrameworkListener::onDataAvailable()
4. FrameworkListener::dispatchCommand()
5. FrameworkCommand::runCommand()—-CommandListener::VolumeCmd::runCommand()
6. VolumeManager::mountVolume()
7. Volume::mountVol()

Process events from kernel

处理kernel发出的挂载事件：

1. SocketListener::runListener <—listening events from kernel
2. NetlinkListener::onDataAvailable()
3. NetlinkHandler::onEvent()
4. VolumeManager::handleBlockEvent , VolumeManager::handleSwitchEvent , VolumeManager::handleUsbCompositeEvent
5. DirectVolume::handleBlockEvent()
6. DirectVolume::handleDiskAdded, DirectVolume::handlePartitionAdded, DirectVolume::handleDiskRemoved, DirectVolume::handlePartitionRemoved,DirectVolume::handleDiskChanged, DirectVolume::handlePartitionChanged,or ignore non add/remove/change event

Reference

• http://blog.csdn.net/qianjin0703/article/details/6362389
• http://www.cnblogs.com/iceocean/articles/1594195.html
• http://www.chinaunix.net/jh/4/822500.html
• http://blog.csdn.net/fudan_abc/article/details/1768277
• http://dongyulong.blog.51cto.com/1451604/389159
• http://blog.csdn.net/datangsoc/article/details/5928132