Oracle Linux - import / load a docker image

When running Docker on Oracle Linux (or any other distro) you can pull images from a registry. The pull command will contact the registry and download the needed. You can use a public registry such as hub.docker.com or you can have a private registry within your secured environment. This mechanism works fine and you should always strive to have the option to use a registry, being it private or public. However, in some cases you do not have the luxury of having a registry and you will have to import / load an image directly into Docker to be able to start a container based upon the image.

When you want to export an image and import it on another Docker node the following commands should be used.

To save an image use the following command:

docker save -o {save image to path} {image name}

To load an image use the following command:

docker load -i {path to image tar file}

It is important to understand you need to use the save and load commands and not the export and import commands as they do very different things:

export - Export a container's filesystem as a tar archive
import - Import the contents from a tarball to create a filesystem image
save - Save one or more images to a tar archive (streamed to STDOUT by default)
load - Load an image from a tar archive or STDIN

Oracle Linux - Set line numbers in VI by default

Without fail almost all Linux distributions come with an editor, vi, that can be used from the command line. Most people who spend time on the Linux command line will be able to use vi. One of the things that can be done in vi is setting line numbers which is extremely handy when you are working on code from the command line. When you do want to set line number on you can do so directly from within vi.

Setting line numbers on can be done using:

:set nu

However when you need to do this often setting the line number to on every time you open vi can become annoying. You would like to have line numbers on by default. The easy way to do so is by ensuring you have a local setting file for vi in your home directory, the file should be named: .vimrc

adding the below content to .vimrc in you home directory will ensure that line numbers are always on when you start vi under Oracle Linux.

[root@localhost ~]# cat ~/.vimrc 
set number
[root@localhost ~]# 

This is however only one of many settings you can do to change the default behaviour of vi. When working a lot with vi you might want to dive a bit into the options to influence the default behaviour as it can save you a lot of time. 

Oracle Linux - Redis & transparent hugepage settings

When deploying Redis on Oracle Linux you might run into a warning situation making a statement about transparent hugepage settings. The statement made by Redis is that Redis might have performance issues when working on a system that is configured to use transparent hugepages (THP). THP is an abstraction layer that automates most aspects of creating, managing, and using huge pages in your memory. If you want to be compliant with the best practices from Redis you will have to take notice if this warning level during startup.

In general the statement made by Redis is the following you will see:

Transparent huge pages must be disabled from your kernel. Use echo never > /sys/kernel/mm/transparent_hugepage/enabled to disable them, and restart your Redis process.

As you can see the statement also includes a clear message on how to resolve this. To be able to see if things go as expected you will have to understand how the content of this file is represented. In general you will have a number of values that are valid:

always
Transparent hugepages will always be used by Oracle Linux

madvise
Oracle Linux will ask advice from madvise for advice if transparent hugepages should be used or not. The madvise() system call is used to give advice or directions to the kernel about the address range beginning at address addr and with size length bytes In most cases, the goal of such advice is to improve system or application performance.

never
Transparent hugepages will never be used by Oracle Linux

If you check the file in question you will see that the choice made is represented by the value between brackets. In our case we have selected never and as you can see in the below example never is between brackets.

[root@localhost tmp]# cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]
[root@localhost tmp]# 

Understanding which value is active, in our case never, is vital for understanding the current setting in your system. Do note, this holds you might want to build a check which is just a bit more intelligent than doing a cat on the file.

Oracle linux - building small Docker containers while using yum

Docker and more precise Docker containers are in general small. The rule for Docker containers is, keep them as small as possible. Building containers is a process where you have to look every step of the way on what you really need and what you can remove or clean. When building a container most people will depend on a mainstream base image, in our case we base most of our containers on the Oracle Linux 7 slim image which is the smallest Docker base image build upon Oracle Linux 7.

One of the rules when you build your own containers on top of the Oracle Linux 7 slim base image (rule applies for all others as well) is that you have to clean after you build. In one example we install a lot of things using yum which are needed as part of the build process. Rule is that you have to clean after you are done to ensure the resulting container is small and is not carrying unneeded things.

in our example we have totally in the beginning a run command, as shown below, that will install a number of packages during the build phase:

# Install dependencies
RUN yum -y install make gcc cc tar gzip

Right after this we initiate a number of compile and build steps. However, after those are done we can remove the installed dependencies again.

# Cleanup dependencies
RUN yum -y erase gcc cc tar gzip

Even though this is good housekeeping it is not enough to ensure your image is small. Yum will keep some files in cache which you also have to ensure are cleaned. If this is not done you will have a lot of unneeded things in your local yum cache which will make your docker image for a container much bigger than needed.

To clean the cache you will also have to ensure you have the following run command in your Dockerfile;

# clean the YUM cache.
RUN yum clean all

Making sure you remove the unneeded dependencies you installed at the end of your Dockerfile and also ensure that you can clean your yum cache will make your image shrink and will ensure you have small Docker images and Docker containers with Oracle Linux 7 in your environment. 

Oracle Linux - getting newer versions of Docker via yum

When using Oracle Linux to run Docker you might depend by default on the Oracle YUM repository. While this is perfectly fine, the Oracle repository might be running behind with the mainstream Docker versions and you will not by default get the latest versions of Docker. As Docker introduces a lot of new features per version and you want to be using the latest stable version the way to go is to use a Docker owned repository.

For Oracle Linux 6 there is a specific repository provided by Docker. you can use this as an addition to the standard Oracle Linux repositories. The example below showcases a docker.repo file located in /etc/yum.repos.d/

[dockerrepo]
name=Docker Repository
baseurl=https://yum.dockerproject.org/repo/main/oraclelinux/6/
enabled=1
gpgcheck=1
gpgkey=https://yum.dockerproject.org/gpg

If you have this file available YUM will be checking this repository when you want to install (or update) docker to a newer version, which in some cases is a newer version than would be availabel via the public-yum.oracle.com repository.