Linux Praktikumstermin 2025_06: Simple Container & btrfs

To show that you have successfully finished this lab, please upload a log file to ILIAS. The log file will contain all commands that you typed in during your session and all system outputs.

It is not a problem if there are commands in the logfile that did not work on first try or were typed in wrong, we just want to see that at some point all the required commands were used correctly.

Open a terminal in the virtual machine and start the logging by using the script command (option -a means: append to file in case the file is non-empty when the logging to file is started):

The following output should appear:

From now on, please only use the console in which the logger is running, so that everything gets logged correctly!

If you want to interrupt the lab and shut down the virtual machine, e.g. to take a break, or if you get interrupted somehow, you have to restart the logging when you continue. Choose a new / different file name. Finally, use a text editor to merge the resulting log files into a single file in the correct order.

After completing the lab, upload the complete result files to the Ilias course.

Install the btrfs file system driver and the tools required for btrfs using the software package btrfs-progs:

Create a btrfs file system using the two hard drives sdb and sdc as devices:

Have the system output information about the file system. Each of the two devices can be used as "reference" to the overall file system spanning the two devices:

Create a directory /mnt/prakt and mount the btrfs file system with administrator rights at this mount point. Again, each of the two devices can be used as reference to the file system.

Make the mounted file system writable for all users and copy some "dummy files" there:

Now that the file system is mounted, the mount point can also in many cases be used as a reference to the file system. E.g. the following command displays information about the file system "level of fill up" analogous to the "classic" df command:

Add another device:

Now we remove one of the original devices:

Create a new subvolume SV1, in the /mnt/prakt directory, i.e. at the top level within the btrfs file system:

Together with the subvolume the associated directory is also created. The directory is still empty, of course:

Make the directory writable for all users and create two files there (file content does not matter, can also be empty):

Place a second, nested subvolume:

Create two files file11.txt and file12.txt in this subvolume.

Place two mount points for the two subvolumes:

Mount the outer / upper subvolume SV1. Again, one of the involved Devices must be specified as a reference to the file system containing the subvolume. Observe that when mounting the outer subvolume, the inner / nested subvolume SV11 is also automatically available:

Now mount the inner subvolume SV11 separately. This shows that each subvolume can also be mounted separately. And it is not a problem that SV11 is now available via "multiple paths" in the file system:

Create a snapshot of the current state of the subvolume SV11. The snapshot is also made available as a subvolume. We need to specify where this new subvolume should be placed in the btrfs file system:

Create a mount point for the new subvolume and mount it there:

The snapshot is writable: delete the file file11.txt from the snapshot. Check that this file is really deleted in the snapshot, but is still available in the source subvolume SV11.

Then delete the file file12.txt in the source subvolume SV11 and check that this file is still available in the snapshot.

Now a backup of the snapshot could now be made, even if it takes a long time, and the original subvolume SV11 could still continue to be in use and could be changed without jeopardizing the consistency of the backup’s data.

Unmount the snapshot:

Now delete the snapshot itself:

Check that the snapshot is no longer available:

Now delete the mount point that is no longer needed:

Now also unmount the other mount:

Download the following file into the directory /home/itsadmin using wget:

https://github.com/ericchiang/containers-from-scratch/releases/download/v0.1.0/rootfs.tar.gz

Copy the file /home/itsadmin/rootfs.tar.gz to the /mnt/prakt directory, i.e. to the btrfs file system.

Unzip the downloaded file with administrator rights (important):

Create a new subvolume root_subvolume:

Move all contents of the rootfs/ directory into the new subvolume root_subvolume and delete the emptied source directory:

Take a look at and remember how much space for data is occupied in the btrfs file system. The rootfs.tar.gz file is about 260 MB in size, the resulting files occupy about 705 MB, altogether under 1 GB:

Now we create two writable snapshots. These are then used as file systems for two containers. Because of the Copy-on-Write principle, the containers do not get real copies of the files, so little space is consumed per container. The original subvolume and its contents are not changed by the containers:

Take a look at and compare with the previous value, how much space for data is occupied in the btrfs file system:

In case of real copying of all files, the occupied disk space would have increased by 2x 705 MB …​

Open a new command line or a new command line tab and start a "simple container" there:

Use ls / to list the contents of the root directory of the container. You should see the files from rootfs there.

In this first container, recursively delete the directory /srv and all of its content.

In the root directory of the container, create a file container1_added.txt. The file content does not matter and the file can also be empty. Create a directory /container1_added_subdir_1 and in this directory create a file /container1_added_subdir_1/container1_added.txt.

Now open another new command line or a new command line tab and start a second "simple container" there:

Also in this container, use ls / to list the contents of the root directory of the container. You should see the files from rootfs there.

In this second container, recursively delete the directory /media including all of its content. Check that the directory is indeed deleted. Also check that the /srv directory still exists, i.e. its deletion in container1 obviously had no effect on container2.

In the root directory of the container, create a file container2_added.txt. The file content does not matter and can also be empty. Create a directory /container2_added_subdir_1 and in this directory a file /container2_added_subdir_1/container2_added.txt.

Change to the command lines of container1 and container2 and check that the changes made in each container did not have an impact on the other container.

Switch to the command line of container2. End this command line and thus the container via exit.

Also end the command line or command line tab that was opened before starting the container container2 by means of exit.

Switch to the command line of container1. Via command ps aux take a look at the list of processes in the container and end this command line and thus the container via exit.

Also end the command line or command line tab that was opened for starting the container by means of exit.

Starting from /mnt/prakt, take a look at the top level content of directory root_subvolume. The changes made in the containers should have had no impact on the contents of this subvolume.

Analyze the top level contents of the directories root_container1 and root_container2. The changes made in the containers should be visible here.

Check the amount of occupied space in the file system:

The amount of occupied space should not have changed significantly.

When you have finalized your lab work, you need to end the logging by entering exit into your console. The following output should appear:

Start a webbrowser inside the virtual machine and upload the log file to ILIAS, using this web browser.

Shut down the VM, remove the VM completely (delete all files) and end your session.