Difference between save and export in Docker

最新推荐文章于 2024-06-02 22:00:10 发布

zhanglizhuo

最新推荐文章于 2024-06-02 22:00:10 发布

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本文通过实际操作对比了Docker中save与export两个命令的不同之处，save用于保存整个镜像及其历史记录，而export则导出容器的当前状态为扁平文件系统。

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I recently played around with Docker, an application container and virtualization‎ technology for Linux. It was pretty cool and I was able to create Docker images and containers within a couple of minutes. Everything was working right out of the box!

At the end of my day I wanted to persist my work. I stumbled over the Docker commands save and export and wondered what their difference is. So I went to StackOverflow and asked a question which was nicely answered by mbarthelemy. Here is what I found out.

How Docker works (in a nutshell)

Docker is based on so called images. These images are comparable to virtual machine images and contain files, configurations and installed programs. And just like virtual machine images you can start instances of them. A running instance of an image is called container. You can make changes to a container (e.g. delete a file), but these changes will not affect the image. However, you can create a new image from a running container (and all it changes) using docker commit <container-id> <image-name>.

Let’s make an example:

# we pull a base Docker image called busybox
# just like in the official Hello-World-example
sudo docker pull busybox

# let's check which images we have
# we should see the image busybox
sudo docker images

# now we make changes to a container of this image
# in this case we make a new folder
sudo docker run busybox mkdir /home/test

# let's check which containers we now have
# note that the container stops after each command
# we should see a busybox container with our command
sudo docker ps -a

# now we can commit this changed container
# this will create a new image called busybox-1
# you see the <CONTAINER ID> with the command above
sudo docker commit <CONTAINER ID> busybox-1

# let's check which images we have now
# we should see the image busybox and busybox-1
sudo docker images

# to see the difference between both images we
# can use the following check for folders:
sudo docker run busybox [ -d /home/test ] && echo 'Directory found' || echo 'Directory not found'
sudo docker run busybox-1 [ -d /home/test ] && echo 'Directory found' || echo 'Directory not found'

# we pull a base Docker image called busybox

# just like in the official Hello-World-example

sudo docker pull busybox

# let's check which images we have

# we should see the image busybox

sudo docker images

# now we make changes to a container of this image

# in this case we make a new folder

sudo docker run busybox mkdir / home / test

# let's check which containers we now have

# note that the container stops after each command

# we should see a busybox container with our command

sudo docker ps - a

# now we can commit this changed container

# this will create a new image called busybox-1

# you see the <CONTAINER ID> with the command above

sudo docker commit < CONTAINER ID > busybox - 1

# let's check which images we have now

# we should see the image busybox and busybox-1

sudo docker images

# to see the difference between both images we

# can use the following check for folders:

sudo docker run busybox [ - d / home / test ] && echo 'Directory found' || echo 'Directory not found'

sudo docker run busybox - 1 [ - d / home / test ] && echo 'Directory found' || echo 'Directory not found'

Now we have two different images (busybox and busybox-1) and we have a container made from busybox which also contains the change (the new folder /home/test). Let’s see how we can persist our changes.

Export

Export is used to persist a container (not an image). So we need the container id which we can see like this:

1	sudo docker ps - a

To export a container we simply do:

1	sudo docker export < CONTAINER ID > > / home / export . tar

The result is a TAR-file which should be around 2.7 MB big (slightly smaller than the one from save).

Save

Save is used to persist an image (not a container). So we need the image name which we can see like this:

1	sudo docker images

To save an image we simply do:

1	sudo docker save busybox - 1 > / home / save . tar

The result is a TAR-file which should be around 2.8 MB big (slightly bigger than the one from export).

The difference

Now after we created our TAR-files, let’s see what we have. First of all we clean up a little bit – we remove all containers and images we have right now:

# first we see which containers we have

sudo docker ps - a

# now we remove all of them

sudo docker rm < CONTAINER ID >

# now we see which images we have

sudo docker images

# and we remove them too

sudo docker rmi busybox - 1

sudo docker rmi busybox

We start with our export we did from the container. We can import it like this:

# import the exported tar ball:

cat / home / export . tar | sudo docker import - busybox - 1 - export : latest

# check the available images

sudo docker images

# and check if a new container made from this image

# contains our folder (it does!)

sudo docker run busybox - 1 - export [ - d / home / test ] && echo 'Directory found' || echo 'Directory not found'

We can do the same for the saved image:

# import the exported tar ball:

docker load < / home / save . tar

# check the available images

sudo docker images

# and check if a new container made from this image

# contains our folder (it does!)

sudo docker run busybox - 1 [ - d / home / test ] && echo 'Directory found' || echo 'Directory not found'

So what’s the difference between both? Well, as we saw the exported version is slightly smaller. That is because it is flattened, which means it lost its history and meta-data. We can see this by the following command:

1 2	# shows the layers of all images sudo docker images -- tree

If we run the command we will see an output like the following. As you can see there, the exported-imported image has lost all of its history whereas the saved-loaded image still have its history and layers. This means that you cannot do any rollback to a previous layer if you export-import it while you can still do this if you save-load the whole (complete) image (you can go back to a previous layer by using docker tag <LAYER ID> <IMAGE NAME>).