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参数dm.fs可以指定容器的rootfs的文件系统，但只支持ext4/xfs：
func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error) {
...
case "dm.fs":
if val != "ext4" && val != "xfs" {
return nil, fmt.Errorf("Unsupported filesystem %s\n", val)
}
devices.filesystem = va这是为什么呢？下面是docker dm的维护者Alexander Larsson的描述：
Additionally we ensure that DISCARD support is enabled in the filesystem so that any files removed in the conttainer filters down to the loopback file making it sparse again.
参考这里
一句话，因为ext4/xfs支持DISCARD。这样，如果容器中删除了文件，空间就会马上还给Thin pool，因为Thin provisioning是支持DISCARD操作的。但是，默认情况下Thin pool是底层是稀疏文件/var/lib/docker/devicemapper/devicemapper/data，所以，只有Host的文件系统支持DISCARD，才能保证稀疏文件空间释放。
# docker images
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
dbyin/tlinux1.2 latest 8297f05d459f 41 hours ago 399.6 MB
dbyin/httpd latest 93e711fab1c1 7 weeks ago 412.7 MB
centos latest 61038e6e3195 3 months ago 236.4 MB
我们可以查看稀疏文件的真正大小：
# ls -lsh /var/lib/docker/devicemapper/devicemapper/data
1.6G -rw------- 1 root root 200G Nov 12 11:52 /var/lib/docker/devicemapper/devicemapper/data
# dmsetup status
yy_pool: 0 409600 thin-pool 0 13/65536 0/3200 - rw no_discard_passdown
docker-8:1-696417-base: 0 41943040 thin 928768 41943039
docker-8:1-696417-pool: 0 419430400 thin-pool 73 633/524288 26115/3276800 - rw no_discard_passdown
我们删除一个image
# docker rmi dbyin/httpd
可以看到稀疏文件并没有变小：
# ls -lsh /var/lib/docker/devicemapper/devicemapper/data
1.6G -rw------- 1 root root 200G Nov 12 11:52 /var/lib/docker/devicemapper/devicemapper/data
# dmsetup status
docker-8:1-696417-base: 0 41943040 thin 928768 41943039
docker-8:1-696417-pool: 0 419430400 thin-pool 73 490/524288 18758/3276800 - rw no_discard_passdown
no_discard_passdown表示dm层不会将DISCARD传给底层的设备(loopback device)，只删除映射关系。
[root@yy3 ~]# dmsetup status
docker-253:1-8790943-pool: 0 209715200 thin-pool 339 876/524288 32432/1638400 - rw discard_passdown queue_if_no_space
可以看到这里为discard_passdown，表示dm会将DISCARD传给底层设备(loopback device)，queue_if_no_space表示如果thin pool没有空闲空间后，IO请求会被排队。另外，error_if_no_space表示如果thin pool没有空闲空间后，直接报错。
[root@yy3 ~]# ls -slh /var/lib/docker/devicemapper/devicemapper/data
2.5G -rw-------. 1 root root 100G Nov 12 06:14 /var/lib/docker/devicemapper/devicemapper/data
[root@yy3 ~]# docker rmi dbyin/httpd
[root@yy3 ~]# ls -slh /var/lib/docker/devicemapper/devicemapper/data
2.0G -rw-------. 1 root root 100G Nov 12 06:15 /var/lib/docker/devicemapper/devicemapper/data
可以看到删除image前后，稀疏文件大小的变化。
docker还提供这个参数，默认值为true，即删除image后，会调用DISCARD，真正释放HOST上空间。
func (devices *DeviceSet) deleteDevice(info *DevInfo) error {
if devices.doBlkDiscard {
// This is a workaround for the kernel not discarding block so
// on the thin pool when we remove a thinp device, so we do it
// manually
if err := devices.activateDeviceIfNeeded(info); err == nil {
if err := BlockDeviceDiscard(info.DevName()); err != nil {
log.Debugf("Error discarding block on device: %s (ignoring)", err)
}
}
}
...
}
func BlockDeviceDiscard(path string) error {
...
if err := ioctlBlkDiscard(file.Fd(), 0, size); err != nil {
return err
}
...
}
func ioctlBlkDiscard(fd uintptr, offset, length uint64) error {
var r [2]uint64
r[0] = offset
r[1] = length
if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, fd, BlkDiscard, uintptr(unsafe.Pointer(&r[0]))); err != 0 {
return err
}
return nil
}
https://www.kernel.org/doc/Documentation/device-mapper/thin-provisioning.txt https://github.com/docker/docker/tree/master/daemon/graphdriver/devmapper
Docker内部存储结构（devicemapper）解析（续）
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原文地址：http://www.cnblogs.com/itcomputer/p/5667797.html
