u 盘会当成 usb storage 在 Settings Storage 里面显示。

内核需支持 usb host，需支持 FUSE 文件系统，在 FS 选项里面配置即可（CONFIG_FUSE_FS=y）。

    mkdir /storage/udisk0 0000 system system
    mkdir /mnt/media_rw/udisk0 0700 media_rw media_rw

service fuse_udisk0 /system/bin/sdcard -u 1023 -g 1023 -w 1023 -d /mnt/media_rw/udisk0 /storage/udisk0
    class late_start
    disabled

配置 fstab.<board>

/block/sda      /mnt/media_rw/udisk0  vfat  defaults  voldmanaged=udisk0:auto

配置 overlay

   <storage android:mountPoint="/storage/udisk0"
             android:storageDescription="@string/storage_usb"
             android:primary="false"
             android:removable="true" />

挂载过程：

/dev/block/vold/8:1 /mnt/media_rw/udisk0 vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/fuse /storage/udisk0 fuse rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0

root@android:/ # logcat -s Vold MountService
--------- beginning of /dev/log/main
--------- beginning of /dev/log/system
I/Vold    (  820): Vold 2.1 (the revenge) firing up
D/Vold    (  820): Volume sdcard1 state changing -1 (Initializing) -> 0 (No-Media)
D/Vold    (  820): Volume sdcard1 state changing -1 (Initializing) -> 0 (No-Media)
D/Vold    (  820): Volume udisk0 state changing -1 (Initializing) -> 0 (No-Media)
D/Vold    (  820): Volume udisk0 state changing 0 (No-Media) -> 2 (Pending)
D/Vold    (  820): Volume udisk0 state changing 2 (Pending) -> 1 (Idle-Unmounted)
D/Vold    (  820): Volume sdcard1 state changing 0 (No-Media) -> 2 (Pending)
D/Vold    (  820): Volume sdcard1 state changing 2 (Pending) -> 1 (Idle-Unmounted)
D/MountService( 1587): got storage path: /storage/sdcard0 description: Internal storage primary: true removable: false emulated: true mtpReserve: 50 allowMassStorage: false maxFileSize: 0
D/MountService( 1587): addVolumeLocked() StorageVolume:
D/MountService( 1587):     mStorageId=65537 mPath=/storage/emulated/0 mDescriptionId=17040662 
D/MountService( 1587):     mPrimary=true mRemovable=false mEmulated=true mMtpReserveSpace=50 
D/MountService( 1587):     mAllowMassStorage=false mMaxFileSize=0 mOwner=UserHandle{0} mUuid=null 
D/MountService( 1587):     mUserLabel=null mState=null 
D/MountService( 1587): got storage path: /storage/sdcard1 description: SD card primary: false removable: true emulated: false mtpReserve: 0 allowMassStorage: false maxFileSize: 4294967296
D/MountService( 1587): addVolumeLocked() StorageVolume:
D/MountService( 1587):     mStorageId=0 mPath=/storage/sdcard1 mDescriptionId=17040663 mPrimary=false 
D/MountService( 1587):     mRemovable=true mEmulated=false mMtpReserveSpace=0 mAllowMassStorage=false 
D/MountService( 1587):     mMaxFileSize=4294967296 mOwner=null mUuid=null mUserLabel=null mState=null 
D/MountService( 1587): got storage path: /storage/udisk0 description: USB storage primary: false removable: true emulated: false mtpReserve: 0 allowMassStorage: false maxFileSize: 0
D/MountService( 1587): addVolumeLocked() StorageVolume:
D/MountService( 1587):     mStorageId=0 mPath=/storage/udisk0 mDescriptionId=17040664 mPrimary=false 
D/MountService( 1587):     mRemovable=true mEmulated=false mMtpReserveSpace=0 mAllowMassStorage=false 
D/MountService( 1587):     mMaxFileSize=0 mOwner=null mUuid=null mUserLabel=null mState=null 
W/MountService( 1587): Duplicate state transition (unmounted -> unmounted) for /storage/sdcard1
D/MountService( 1587): volume state changed for /storage/sdcard1 (unmounted -> removed)
W/MountService( 1587): Duplicate state transition (unmounted -> unmounted) for /storage/udisk0
I/Vold    (  820): /dev/block/vold/8:1 being considered for volume udisk0
D/Vold    (  820): Volume udisk0 state changing 1 (Idle-Unmounted) -> 3 (Checking)
D/MountService( 1587): volume state changed for /storage/udisk0 (unmounted -> checking)
D/MountService( 1587): sendStorageIntent Intent { act=android.intent.action.MEDIA_CHECKING dat=file:///storage/udisk0 (has extras) } to UserHandle{-1}
I/Vold    (  820): Filesystem check completed OK
D/Vold    (  820): blkid identified as /dev/block/vold/8:1: UUID="402E-0EE3" TYPE="vfat" 
D/Vold    (  820): Volume udisk0 state changing 3 (Checking) -> 4 (Mounted)
D/MountService( 1587): volume state changed for /storage/udisk0 (checking -> mounted)
D/MountService( 1587): sendStorageIntent Intent { act=android.intent.action.MEDIA_MOUNTED dat=file:///storage/udisk0 (has extras) } to UserHandle{-1}
W/MountService( 1587): Duplicate state transition (mounted -> mounted) for /storage/emulated/0

Vold 解析 fstab 文件

在 Android 4.4 中， Vold 会解析 vold.<hardware> 文件

   property_get("ro.hardware", propbuf, "");
    snprintf(fstab_filename, sizeof(fstab_filename), FSTAB_PREFIX"%s", propbuf);

    fstab = fs_mgr_read_fstab(fstab_filename);
    if (!fstab) {
        SLOGE("failed to open %s\n", fstab_filename);
        return -1;
    }

其中 #define FSTAB_PREFIX "/fstab."  所以，配置的 fatab 文件必须放在根目录， Vold 是 android 的后台进程，将会一直监听 fatab 里面标记为 voldmanaged 的项目

如果没有标记，则略过。没有标记的项目。可以交给 init 的 mount 命令处理，但是不会监听，所以你可以将recovery 和 cache 目录这样只需启动是挂载的目录项放在 fatab

里面交给 init 的 mount 命令处理，例如三星 device/samsung/manta/fstab.manta 

# Android fstab file.
#<src>                                                  <mnt_point>         <type>    <mnt_flags and opt
ions>                       <fs_mgr_flags>
# The filesystem that contains the filesystem checker binary (typically /system) cannot
# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK


/dev/block/platform/dw_mmc.0/by-name/system     /system             ext4      ro
                            wait
/dev/block/platform/dw_mmc.0/by-name/cache      /cache              ext4      noatime,nosuid,nodev,nombl
k_io_submit,errors=panic    wait,check
/dev/block/platform/dw_mmc.0/by-name/userdata   /data               ext4      noatime,nosuid,nodev,nombl
k_io_submit,errors=panic    wait,check,encryptable=/dev/block/platform/dw_mmc.0/by-name/metadata
/dev/block/platform/dw_mmc.0/by-name/efs        /factory            ext4      noatime,nosuid,nodev,ro
                            wait
/dev/block/platform/dw_mmc.0/by-name/boot       /boot               emmc      defaults
                            defaults
/dev/block/platform/dw_mmc.0/by-name/recovery   /recovery           emmc      defaults
                            defaults
/dev/block/platform/dw_mmc.0/by-name/misc       /misc               emmc      defaults
                            defaults
/dev/block/mmcblk0boot0                         /bootloader         emmc      defaults
                            defaults

在系统启动调用 mount ：

on fs
    mkdir /factory 0775 radio radio

    mount_all /fstab.manta

同理， init 的 mount 也会忽略标记为 voldmanaged 的项目

init 的 mount 命令与 vold 的 mount 管理大有不同，两者不能混用。

MountService 与 Vold 会建立 socket 通信，具体的挂载操作都是下发命令，交给 Vold 去完成。 MountService 会解析我们配置的 storage_list.xml 

   private void readStorageListLocked() {
        mVolumes.clear();
        mVolumeStates.clear();

        Resources resources = mContext.getResources();

        int id = com.android.internal.R.xml.storage_list;
        XmlResourceParser parser = resources.getXml(id);
        AttributeSet attrs = Xml.asAttributeSet(parser);

        try {
            XmlUtils.beginDocument(parser, TAG_STORAGE_LIST);
            while (true) {
                XmlUtils.nextElement(parser);

Vold 获取到 list 里面的设备变化时将会发送 state change event， MountService 在 onEvent 处理各种不同 state ，其中获取到设备插入，将会执行挂载操作：

            if (code == VoldResponseCode.VolumeDiskInserted) {
                new Thread("MountService#VolumeDiskInserted") {
                    @Override
                    public void run() {
                        try {
                            int rc;
                            if ((rc = doMountVolume(path)) != StorageResultCode.OperationSucceeded) {
                                Slog.w(TAG, String.format("Insertion mount failed (%d)", rc));
                            }
                        } catch (Exception ex) {
                            Slog.w(TAG, "Failed to mount media on insertion", ex);
                        }
                    }
                }.start();

doMountVolume  只是发送 mount 命令给 Vold 

   private int doMountVolume(String path) {
        int rc = StorageResultCode.OperationSucceeded;

        final StorageVolume volume;
        synchronized (mVolumesLock) {
            volume = mVolumesByPath.get(path);
        }

        if (DEBUG_EVENTS) Slog.i(TAG, "doMountVolume: Mouting " + path);
        try {
            mConnector.execute("volume", "mount", path);

最终 Vold 里面执行相应的 mount 操作，我们的 u 盘是 vfat 格式，我只测试了这个格式， Linux 下的格式没有测试，因为 Vold 只支持 fat 和 ext4

int VolumeManager::mountVolume(const char *label) {
    Volume *v = lookupVolume(label);

    if (!v) {
        errno = ENOENT;
        return -1;
    }

    return v->mountVol();
}

       if (Fat::doMount(devicePath, getMountpoint(), false, false, false,
                AID_MEDIA_RW, AID_MEDIA_RW, 0007, true)) {
            SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));
            continue;