Deep Dive Images and Containers
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Image Layer
Union Mounts
Ability to mount multiple file systems on top of each other.
If we are having same file with different data in multiple
layers, then top layer data overwrites all.
When we run a container, whatever we write it forms the
uppermost layer.
IMAGE Union mounts 2
Image Layer and OverlayFS
#docker pull ubuntu
Using
default tag: latest
latest:
Pulling from library/ubuntu
d54efb8db41d:
Pull complete
f8b845f45a87:
Pull complete
e8db7bf7c39f:
Pull complete
9654c40e9079:
Pull complete
6d9ef359eaaa:
Pull complete
Digest:
sha256:dd7808d8792c9841d0b460122f1acf0a2dd1f56404f8d1e56298048885e45535
Status:
Downloaded newer image for ubuntu:latest
Each image layer has its own directory
under /var/lib/docker/overlay/. This is where the contents of each image
layer are stored.
The output of the command below shows the five
directories that store the contents of each image layer just pulled. However,
as can be seen, the image layer IDs do not match the directory names in .
[root@docker overlay]# ls -ltr
total 0
drwx------.
3 root root 18 Apr 1 07:44
0e2c57fb6e308cbef7250967f66fd14c8d49f4422e758ce9cd0b2d3f05b3e771
drwx------.
3 root root 18 Apr 1 07:44
2d0d4233891dd532127336c470ec8cb41d0b95d140ee946a73b336530a742d5f
drwx------.
3 root root 18 Apr 1 07:44
0179b7a9394b83d47f88fe23abe0e9fa529c245506434fc32f1ea39179898737
drwx------.
3 root root 18 Apr 1 07:44
43696dfe7b0b4d5d77e94a3747435498a3819fecec846e91c2f48c34bd1c2428
drwx------.
3 root root 18 Apr 1 07:44
90c0ed5454b33d404b5600ef02c5efa78dc5b470cf03b2005a34ca489883f772
Now make a change in a container and find it on the
overlayFS.
#docker run -it 0ef2e08ed3fa
0ef2e08ed3fa #apt-get update
0ef2e08ed3fa #apt-get install vim
0ef2e08ed3fa #vi /tmp/testfile
This is
a test file.
Now
press “Ctrl P Q”
# mount | grep overlay
/dev/mapper/cl-root
on /var/lib/docker/overlay type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
overlay
on
/var/lib/docker/overlay/0cab383e563082ef352c77e2eb186f136fdd484b6a98a0c4edb8e7caac231f07/merged type overlay
(rw,relatime,seclabel,lowerdir=/var/lib/docker/overlay/90c0ed5454b33d404b5600ef02c5efa78dc5b470cf03b2005a34ca489883f772/root,upperdir=/var/lib/docker/overlay/0cab383e563082ef352c77e2eb186f136fdd484b6a98a0c4edb8e7caac231f07/upper,workdir=/var/lib/docker/overlay/0cab383e563082ef352c77e2eb186f136fdd484b6a98a0c4edb8e7caac231f07/work)
The “lower-id”
file contains the ID of the top layer of the image the container is based on.
This is used by OverlayFS as the “lowerdir”.
The “upper”
directory is the containers read-write layer. Any changes made to the container
are written to this directory.
The “merged”
directory is effectively the containers mount point. This is where the unified
view of the image (“lowerdir”) and container (“upperdir”) is exposed. Any
changes written to the container are immediately reflected in this directory.
The “work” directory is required for OverlayFS to function.
It is used for things such as copy_up operations.
# cat
/var/lib/docker/overlay/0cab383e563082ef352c77e2eb186f136fdd484b6a98a0c4edb8e7caac231f07/merged/tmp/testfile
this is
a testfile
Copying Image to other hosts
Now stop the above container
# docker ps
CONTAINER ID
IMAGE COMMAND CREATED STATUS PORTS NAMES
d956f56f2ac5 0ef2e08ed3fa "/bin/bash" 19 minutes ago Up 19 minutes angry_wilson
Take the container id from above and stop it
# docker stop
d956f56f2ac5
d956f56f2ac5
After stopping commit it and give it a name.
# docker commit
d956f56f2ac5 testfile
sha256:fa914368ee56dae3b5b0c843ac407dc6ed2481b6116aad665df092b1348cf668
# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
testfile latest fa914368ee56 8 seconds ago 228 MB
centos latest 98d35105a391 2 weeks ago 192 MB
ubuntu latest 0ef2e08ed3fa 4 weeks ago 130 MB
So, the size of testfile is increased by 36MB, as we have
updated the server.
Run the history command to check it.
# docker history testfile
IMAGE CREATED CREATED BY SIZE COMMENT
fa914368ee56 10 minutes ago /bin/bash 97.9 MB
0ef2e08ed3fa 4 weeks ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0 B
<missing> 4 weeks ago /bin/sh -c mkdir -p /run/systemd
&& echo '... 7 B
<missing> 4 weeks ago /bin/sh -c sed -i
's/^#\s*\(deb.*universe\... 1.9 kB
<missing> 4 weeks ago /bin/sh -c rm -rf
/var/lib/apt/lists/* 0 B
<missing> 4 weeks ago /bin/sh -c set -xe && echo '#!/bin/sh' >... 745 B
<missing> 4 weeks ago /bin/sh -c #(nop) ADD
file:efb254bc677d66d... 130 MB
So, we started the container by /bin/bash which is
getting reflected here. And what was the size of the change.
Now save the image as tar ball. Here, even if we don’t
mention file name.tar, docker will create tar ball by default.
# docker save -o
/tmp/tesfile.tar testfile
# ls -ltr
/tmp/tesfile.tar
-rw-------.
1 root root 235798528 Apr 1 09:56
/tmp/tesfile.tar
#du -sh
/tmp/tesfile.tar
225M /tmp/tesfile.tar
Transfer the file to other Ubuntu server
# scp
/tmp/tesfile.tar 192.168.56.226:/tmp/
To check all the file inside the tar
# tar -tf
/tmp/tesfile.tar
# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
Load the image
# docker load -i
/tmp/tesfile.tar
9d3227c1793b:
Loading layer 121.3MB/121.3MB
a1a54d352248:
Loading layer 15.87kB/15.87kB
511ddc11cf68:
Loading layer 11.78kB/11.78kB
08f405d988e4:
Loading layer 5.632kB/5.632kB
73e5d2de6e3e:
Loading layer 3.072kB/3.072kB
7738492b326d:
Loading layer 97.87MB/97.87MB
Loaded
image: testfile:latest
# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
testfile latest 00a5cfa72dbd 10 minutes ago 213MB
# docker run -it
testfile /bin/bash
root@4880f241a8f1:/#
cat /tmp/testfile
this is
a testfile
More about Containers
Image: Build time
construct
Container: Run
time construct
- Rootfs is read only
- Kernel is shared by all the containers
- Thin writable layer
One process per Container
We can check all the top processes running on a container
# docker run -it ubuntu
/bin/bash
# docker ps
CONTAINER
ID IMAGE COMMAND CREATED STATUS PORTS NAMES
72f512e7e091 ubuntu "/bin/bash" 9 seconds ago Up 9 seconds dazzling_hypatia
# docker top
72f512e7e091
UID PID PPID C STIME TTY TIME CMD
root 10816 10805 0 10:44 pts/1 00:00:00 /bin/bash
#docker inspect
72f512e7e091
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