Saurav kumar – Best DevOps

Define a docker image in 10 bullets points

Saurav kumar — Tue, 17 Aug 2021 07:33:30 +0000

Docker image is a collection of filesystems. Each image has a unique UUID assigned via sha-256 algorithm.
Each image and a container has storage device assocaited with it.
Each file system represents a layer in the image.
The base layer for any image is the rootFS.
Every layer comes bundled with it’s parent layer.
All the layers of an image are read only.
At the time of container start, a merged layer is created including all the layers of corresponding image and made writable for custimization. Any change to this merged layer, doesn’t affect the actual image layers.
A container is one mounted instance of an image. As many containers from an image will be created, that many merged layers will be created for each container with a unique UUID.
The top layer has the highest precedence i.e. in case of duplicate files in multiple layers, the topmost layers file will be final output in the merged layer.
The default base directory for docker images and containers is /var/lib/docker.
When a container is stopped or killed, the corresponding merged layer will be cleaned from /var/lib/docker folder.

Assignment Day-2 Answer

Saurav kumar — Tue, 17 Aug 2021 05:33:48 +0000

Docker Update command

Docker update command is used to change the configurations of one or more docker container dynamically.

[root@ip-172-31-7-40 centos]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
15c695103fd0 jenkins/jenkins “/sbin/tini — /usr/…” 19 hours ago Exited (137) 7 minutes ago sharp_albattani
e91a38a225ad ubuntu “bash” 20 hours ago Up 16 minutes practical_shirley
[root@ip-172-31-7-40 centos]#
[root@ip-172-31-7-40 centos]# docker inspect 15c695103fd0 | grep -i cpu
“CpuShares”: 100,
“NanoCpus”: 1000000000,
“CpuPeriod”: 0,
“CpuQuota”: 0,
“CpuRealtimePeriod”: 0,
“CpuRealtimeRuntime”: 0,
“CpusetCpus”: “”,
“CpusetMems”: “”,
“CpuCount”: 0,
“CpuPercent”: 0,
[root@ip-172-31-7-40 centos]#

The current CpuShares value for this container is 100. Will update the same using docker update command as below to 10.

[root@ip-172-31-7-40 centos]# docker update -c 10 15c695103fd0
15c695103fd0
[root@ip-172-31-7-40 centos]# docker inspect 15c695103fd0 | grep -i cpu
“CpuShares”: 10,
“NanoCpus”: 1000000000,
“CpuPeriod”: 0,
“CpuQuota”: 0,
“CpuRealtimePeriod”: 0,
“CpuRealtimeRuntime”: 0,
“CpusetCpus”: “”,
“CpusetMems”: “”,
“CpuCount”: 0,
“CpuPercent”: 0,
[root@ip-172-31-7-40 centos]#

Below we will update the pids limit that maximum number of pids we can start in this container.

[root@ip-172-31-7-40 centos]# docker inspect e91a38a225ad| grep -i pid
“Pid”: 2176,
“PidMode”: “”,
“PidsLimit”: 1,
[root@ip-172-31-7-40 centos]# docker update –pids-limit 25 e91a38a225ad
e91a38a225ad
[root@ip-172-31-7-40 centos]#
[root@ip-172-31-7-40 centos]# docker inspect e91a38a225ad| grep -i pid
“Pid”: 2176,
“PidMode”: “”,
“PidsLimit”: 25,
[root@ip-172-31-7-40 centos]#

Docker WAIT command

Docker wait command is used to block untill one or more containers stop and return with the exit code.

Open 2 terminals. Execute docker wait command in 1 terminal with the docker id and it will block. Run docker stop command in other terminal and the waiting terminal should unblock with the exit code.

[root@ip-172-31-7-40 centos]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
15c695103fd0 jenkins/jenkins “/sbin/tini — /usr/…” 20 hours ago Up 5 seconds 8080/tcp, 50000/tcp sharp_albattani
e91a38a225ad ubuntu “bash” 20 hours ago Up 2 seconds practical_shirley
[root@ip-172-31-7-40 centos]#
[root@ip-172-31-7-40 centos]# docker stop 15c695103fd0
15c695103fd0
[root@ip-172-31-7-40 centos]#

[root@ip-172-31-7-40 centos]#
[root@ip-172-31-7-40 centos]# docker wait 15c695103fd0
143
[root@ip-172-31-7-40 centos]#

[root@ip-172-31-7-40 centos]# docker wait e91a38a225ad
137
[root@ip-172-31-7-40 centos]#
[root@ip-172-31-7-40 centos]# docker kill e91a38a225ad
e91a38a225ad
[root@ip-172-31-7-40 centos]#

If a docker container is already stopped or killed, the wait command will return immediately with the error code without blocking. Also, if the container is just paused, wait command won’t come out of block state. Wait command only works if the container is stopped and not on only pause.

Answer for Assignment – Day 1

Saurav kumar — Mon, 16 Aug 2021 10:52:07 +0000

What is the diff between docker pause and docker unpause?

Docker pause command is used to move the processes running within the container to paused state by sending a signal SIGSTOP and then the docker unpause command can be used to bring back the processes to running state.

What is the diff between docker stop and docker kill?

Docker stop command is used to stop all the running processes within the container by sending a SIGTERM signal which can be handled using signal handlers within the process to do any kind of cleanup activities. While docker kill command kills all the processes within a container through SIGKILL command.