// thin device数据结构type DevInfo struct {	Hash          string     `json:"-"`	DeviceId      int        `json:"device_id"`	Size          uint64     `json:"size"`	TransactionId uint64     `json:"transaction_id"`	Initialized   bool       `json:"initialized"`	devices       *DeviceSet `json:"-"`	mountCount int    `json:"-"`	mountPath  string `json:"-"`	lock sync.Mutex `json:"-"`}// thin pool数据结构type DeviceSet struct {	MetaData	//根文件夹，默觉得/var/lib/docker/devicemapper	root             string	//创建thin device名字使用的前缀，`docker-${major}:${minor}-${inode}`	devicePrefix     string	TransactionId    uint64	NewTransactionId uint64	nextDeviceId     int	//选项	dataLoopbackSize     int64 	///var/lib/docker /devicemapper/devicemapper/data稀疏文件大小	metaDataLoopbackSize int64	///var/lib/docker/devicemapper/devicemapper/metadata稀疏文件大小	baseFsSize           uint64 	//base image之上格式化的文件系统大小	filesystem           string	//base image之上格式化的文件系统类型	mountOptions         string		mkfsArgs             []string	//格式化base image文件系统时的选项	dataDevice           string	//指定使用哪个设备作为data device，eg。/dev/sda	metadataDevice       string	//指定使用哪个设备作为metadata device，eg。/dev/sda	doBlkDiscard         bool		thinpBlockSize       uint32	//thin pool block size}// devmapper的driver数据结构type Driver struct {	*DeviceSet	home string 		//home默觉得/var/lib/docker/devicemapper}

docker使用device mapper的架构方式：

                                                     

//初始化devicemapper driver//	home=/var/lib/docker/devicemapper//	options=device mapper的选项//	调用路径：newdevice->initfunc1.1 func Init(home string, options []string) (graphdriver.Driver, error) {	//初始化deviceset	deviceSet, err := NewDeviceSet(home, true, options)	if err != nil {		return nil, err	}	...	d := &Driver{		DeviceSet: deviceSet,		home:      home,	}	return d, nil}//初始化deviceset//	device set root=/var/lib/docker/devicemapper//	调用路径：Init->NewDeviceSet1.2 func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error) {	SetDevDir("/dev")	devices := &DeviceSet{		root:                 root,		//metaData通过deviceID存放thin device的配置信息		MetaData:             MetaData{Devices: make(map[string]*DevInfo)},		dataLoopbackSize:     DefaultDataLoopbackSize,		metaDataLoopbackSize: DefaultMetaDataLoopbackSize,		baseFsSize:           DefaultBaseFsSize,		filesystem:           "ext4",		doBlkDiscard:         true,		thinpBlockSize:       DefaultThinpBlockSize,	}	//初始化deviceset选项參数	for _, option := range options {		key, val, err := utils.ParseKeyValueOpt(option)		if err != nil {			return nil, err		}		key = strings.ToLower(key)		switch key {		case "dm.basesize":			size, err := units.RAMInBytes(val)			if err != nil {				return nil, err			}			devices.baseFsSize = uint64(size)		...		default:			return nil, fmt.Errorf("Unknown option %s\n", key)		}	}	//由deviceset继续完毕初始化	if err := devices.initDevmapper(doInit); err != nil {		return nil, err	}	return devices, nil}//	初始化thin pool//		调用路径：NewDeviceSet->initDevmapper1.3 func (devices *DeviceSet) initDevmapper(doInit bool) error {	logInit(devices)	//创建/var/lib/docker/devicemapper/metadata文件夹	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil && !os.IsExist(err) {		return err	}	//获取/var/lib/docker文件夹所在设备的 inode	st, err := os.Stat(devices.root)	if err != nil {		return fmt.Errorf("Error looking up dir %s: %s", devices.root, err)	}	sysSt := st.Sys().(*syscall.Stat_t)	//thin device取名规则docker-$major:$minor-$inode-$imageid/$containerid	//thin poll取名为docker-$major:$minor-$inode-pool	devices.devicePrefix = fmt.Sprintf("docker-%d:%d-%d", major(sysSt.Dev), minor(sysSt.Dev), sysSt.Ino)	//假设thin pool device存在，获取device的信息	utils.Debugf("Checking for existence of the pool '%s'", devices.getPoolName())	info, err := getInfo(devices.getPoolName())	if info == nil {		utils.Debugf("Error device getInfo: %s", err)		return err	}	setCloseOnExec("/dev/mapper/control")	createdLoopback := false	//创建thin pool	if info.Exists == 0 {		utils.Debugf("Pool doesn't exist. Creating it.")		var (			dataFile     *os.File			metadataFile *os.File		)		//没有指定datadevice设备		if devices.dataDevice == "" {			//检查/var/lib/docker/devicemapper/devicemapper/data文件是否存在			hasData := devices.hasImage("data")			//既不要求初始化新的devicemapper，又没有旧的data文件			if !doInit && !hasData {				//返回错误				return errors.New("Loopback data file not found")			}			//创建data loopdevice			if !hasData {				createdLoopback = true			}			//创建/var/lib/docker/devicemapper/devicemapper/data 稀疏文件			data, err := devices.ensureImage("data", devices.dataLoopbackSize)			if err != nil {				utils.Debugf("Error device ensureImage (data): %s\n", err)				return err			}			//data文件与loopback device关联			dataFile, err = attachLoopDevice(data)			if err != nil {				return err			}		} else {			//假设指定了data device，则打开			dataFile, err = os.OpenFile(devices.dataDevice, os.O_RDWR, 0600)			if err != nil {				return err			}		}		defer dataFile.Close()	//通过相同的办法初始化metadata device	...		//创建thin pool	if err := createPool(devices.getPoolName(), dataFile, metadataFile, devices.thinpBlockSize); err != nil {			return err	}	//没有创建新loopback device，则从文件夹/var/lib/docker/devicemapper/metadata/$ids	//载入旧的metadata	if !createdLoopback {		if err = devices.initMetaData(); err != nil {			return err		}	}	//初始化一个新的空镜像文件，作为全部镜像的祖先镜像	if doInit {		if err := devices.setupBaseImage(); err != nil {			utils.Debugf("Error device setupBaseImage: %s\n", err)			return err		}	}	return nil}//	创建祖先镜像1.4 func (devices *DeviceSet) setupBaseImage() error {	//祖先镜像的描写叙述信息存放在/var/lib/docker/devicemapper/metadata/base	oldInfo, _ := devices.lookupDevice("")	//之前已经创建。并完毕了初始化。则直接成功返回	if oldInfo != nil && oldInfo.Initialized {		return nil	}	//已创建。但未完毕初始化，删除base device	if oldInfo != nil && !oldInfo.Initialized {		utils.Debugf("Removing uninitialized base image")		if err := devices.deleteDevice(oldInfo); err != nil {			return err		}	}	//下一个可用的deviceid	id := devices.nextDeviceId	//创建base device	if err := createDevice(devices.getPoolDevName(), &id); err != nil {		return err	}	devices.nextDeviceId = (id + 1) & 0xffffff	//向thin pool注冊base device	utils.Debugf("Registering base device (id %v) with FS size %v", id, devices.baseFsSize)	info, err := devices.registerDevice(id, "", devices.baseFsSize)	if err != nil {		_ = deleteDevice(devices.getPoolDevName(), id)		return err	}	//激活base device	if err = devices.activateDeviceIfNeeded(info); err != nil {		return err	}	//在base device之上格式化新文件系统	if err := devices.createFilesystem(info); err != nil {		return err	}	//完毕初始化，保存metadata到/var/lib/docker/devicemapper/metadata/base中	info.Initialized = true	if err = devices.saveMetadata(info); err != nil {		info.Initialized = false		return err	}	return nil}