Devicemapper 源码分析和流程

创建 thinpool

// This is theprogrammatic example of "dmsetup create"

funccreatePool(poolName string, dataFile, metadataFile *os.File, poolBlockSizeuint32) error {

...

params := fmt.Sprintf("%s %s %d 32768 1skip_block_zeroing", metadataFile.Name(), dataFile.Name(), poolBlockSize)

if err := task.AddTarget(0, size/512,"thin-pool", params); err != nil { return fmt.Errorf("Can‘t addtarget %s", err)

}

...

}

相当于执行下面的操作：

#  dmsetup create docker-8:1-696417-pool--table 0 419430400 thin-pool 7:1 7:0 128 32768 1 skip_block_zeroing‘

# dmsetup table docker-8:1-696417-pool

0 419430400thin-pool 7:1 7:0 128 32768 1 skip_block_zeroing

创建 BaseImage

实际上，thin-provisionedvolume 分两步，首先是发送一个消息给 pool，创建一个 volume。然后激活

volume。只有 activated 的 volume，才能在 dmsetup info 的输出中看到。 （1）Creating a newthinly-provisioned volume

funccreateDevice(poolName string, deviceId *int) error {

…

if err :=task.SetMessage(fmt.Sprintf("create_thin %d", *deviceId)); err != nil{ return fmt.Errorf("Can‘t set message %s", err)

}

相当于执行下面的操作：

#dmsetup message/dev/mapper/ docker-8:1-696417-pool 0 "create_thin 0"

可以看到 base 的 device_id 为 0。

（2）activatedthinly-provisioned volumes

funcactivateDevice(poolName string, name string, deviceId int, size uint64) error {

...

params :=fmt.Sprintf("%s %d", poolName, deviceId)

if err := task.AddTarget(0, size/512,"thin", params); err != nil { return fmt.Errorf("Can‘t addtarget %s", err)

}

相当于执行下面的操作：

#dmsetup createdocker-8:1-696417-base --table "0 41943040 thin /dev/mapper/docker-8:1-696417-pool 0"

#dmsetup table docker-8:1-696417-base 0 41943040 thin 253:0 0

只有 activated 的 volume，才能在 dmsetup info 的输出中看到。 Devicemapper 的基本操作

Driver的基本操作

///清除 thin pool

func (d *Driver)Cleanup()

///当加载新镜像时,添加一个新 thin volume,id 为 containerid 或 imageid func (d *Driver) Create(id, parent string)

///挂载 thin volume 到/var/lib/docker/devicemapper/mnt/$id 目录下(docker start) func (d *Driver) Get(id, mountLabelstring)

///从/var/lib/docker/devicemapper/mnt/$id目录 umount thinvolume(docker stop) func (d *Driver) Put(id string)

///删除 volume(真正删除)

func (d *Driver)Remove(id string)

Thinpool 的基本操作

///在 thin pool 中创建一个新的 snapshot volume

func (devices*DeviceSet) AddDevice(hash, baseHash string)

///删除 thin volume(释放空间,删除(remove+delete)thin volume) func (devices *DeviceSet) DeleteDevice(hashstring)

/// 将 thin volume 从 /var/lib/docker/devicemapper/mnt/$id umount, deactivate(remove )thinvolume(don‘t delete)

func (devices*DeviceSet) UnmountDevice(hash string)

///activate thin volume ，然后 mount 到/var/lib/docker/devicemapper/mnt/$idfunc (devices *DeviceSet) MountDevice(hash, path, mountLabel string)

///thin pool 的统计信息(docker info)

func (devices*DeviceSet) Status() *Status

///thin pool 初始化

funcNewDeviceSet(root string, doInit bool, options []string)

Devmapper接口

devmapper/devmapper.go封装了 OS 层的 thin volume 的基本操作。

///dmsetup suspend

funcsuspendDevice(name string)

///dmsetup resume

funcresumeDevice(name string)

///messagecreate_thin

funccreateDevice(poolName string, deviceId *int)

///message delete

funcdeleteDevice(poolName string, deviceId int)

///dmsetup remove

funcremoveDevice(name string)

///dmsetup create

funcactivateDevice(poolName string, name string, deviceId int, size uint64)

///message‘create_snap‘

funccreateSnapDevice(poolName string, deviceId *int, baseName string, baseDeviceIdint)

三者之间的调用关系如下：

查看 stop 的容器的文件系统

stop 的容器的 thin volume 都处于未激活（deactivate）状态，我们可以将其激活（activate），然后查 看文件系统中的内容。

我们创建一个容器，不启动：

# docker create --name="yy1" centos /bin/bash 93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9

metadata 下面新增两个目录：

# ls metadata/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e993f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9 -init

我们可以查看 thin volume 的信息

#cat metadata/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9{"device_id":5,"size":21474836480,"transaction_id":8,"initialized":false}

# catmetadata/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9 -init{"device_id":4,"size":21474836480,"transaction_id":7,"initialized":false}

我们来尝试手动挂载 thin volume，首先 activate thin volume：

#
dmsetup
create
93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9 -init
--table    "0
41943040 thin 253:0 4"

然后就可以挂载该 thin volume 了：

#

mount

/dev/mapper/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9-init mnt/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9-init

# lsmnt/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9 -init/ id lost+foundrootfs

deactivate thinvolume

# umountmnt/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9-init

# dmsetup remove93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01