Docker启动后，会创建thin pool和base image，如下：
# dmsetup info
Name: docker-8:1-696417-base
State: ACTIVE
Read Ahead: 256
Tables present: LIVE
Open count: 0
Event number: 0
Major, minor: 253, 1
Number of targets: 1
Name: docker-8:1-696417-pool
State: ACTIVE
Read Ahead: 256
Tables present: LIVE
Open count: 1
Event number: 0
Major, minor: 253, 0
Number of targets: 1
/dev/dm-0对应docker-8:1-696417-pool，大小为200G。
# ls /dev/dm-0 -lh
brw-rw---- 1 root disk 253, 0 Nov 7 13:01 /dev/dm-0
# blockdev --report /dev/dm-0
RO RA SSZ BSZ StartSec Size Device
rw 256 512 4096 0 214748364800 /dev/dm-0
/dev/dm-1对应docker-8:1-696417-base，大小为20G。
# ls /dev/dm-1 -lh
brw-rw---- 1 root disk 253, 1 Nov 7 13:04 /dev/dm-1
# blockdev --report /dev/dm-1
RO RA SSZ BSZ StartSec Size Device
rw 256 512 4096 0 21474836480 /dev/dm-1
Devicemapper的初始化
整个流程大体如下：
###创建thin pool
// This is the programmatic example of "dmsetup create"
func createPool(poolName string, dataFile, metadataFile *os.File, poolBlockSize uint32) error {
...
params := fmt.Sprintf("%s %s %d 32768 1 skip_block_zeroing", metadataFile.Name(), dataFile.Name(), poolBlockSize)
if err := task.AddTarget(0, size/512, "thin-pool", params); err != nil {
return fmt.Errorf("Can't add target %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 419430400 thin-pool 7:1 7:0 128 32768 1 skip_block_zeroing
###创建BaseImage 实际上，thin-provisioned volume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 new thinly-provisioned volume
func createDevice(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"
# hexdump -C /var/lib/docker/devicemapper/metadata/base
00000000 7b 22 64 65 76 69 63 65 5f 69 64 22 3a 30 2c 22 |{"device_id":0,"|
00000010 73 69 7a 65 22 3a 32 31 34 37 34 38 33 36 34 38 |size":2147483648|
00000020 30 2c 22 74 72 61 6e 73 61 63 74 69 6f 6e 5f 69 |0,"transaction_i|
00000030 64 22 3a 31 2c 22 69 6e 69 74 69 61 6c 69 7a 65 |d":1,"initialize|
00000040 64 22 3a 74 72 75 65 7d |d":true}|
可以看到base的device_id为0。
（2）activated thinly-provisioned volumes
func activateDevice(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 add target %s", err)
}
相当于执行下面的操作：
#dmsetup create docker-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/$id目录下(docker start)
func (d *Driver) Get(id, mountLabel string)
///从/var/lib/docker/devicemapper/mnt/$id目录umount thin volume(docker stop)
func (d *Driver) Put(id string)
///删除volume(真正删除)
func (d *Driver) Remove(id string)
###Thin pool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(hash string)
///将thin volume从/var/lib/docker/devicemapper/mnt/$id umount, deactivate(remove )thin volume(don't delete)
func (devices *DeviceSet) UnmountDevice(hash string)
///activate thin volume ，然后mount到/var/lib/docker/devicemapper/mnt/$id
func (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
func NewDeviceSet(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
func suspendDevice(name string)
///dmsetup resume
func resumeDevice(name string)
///message create_thin
func createDevice(poolName string, deviceId *int)
///message delete
func deleteDevice(poolName string, deviceId int)
///dmsetup remove
func removeDevice(name string)
///dmsetup create
func activateDevice(poolName string, name string, deviceId int, size uint64)
///message 'create_snap'
func createSnapDevice(poolName string, deviceId *int, baseName string, baseDeviceId int)
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/
93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9
93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9-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}
# cat metadata/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
# ls mnt/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9-init/
id lost+found rootfs
deactivate thin volume
# umount mnt/93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01ad89f8e9-init
# dmsetup remove 93f595ea79a0420cc263d054d3e63b5ad1e4cc3da128167984a6ac01
Docker的devicemapper的维护者Alexander Larsson在这篇文章中详细讨论了devicemapper最初的一些想法。