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Images are the basic building blocks of the Docker, and are organized in a layered manner. The images are utilized as read-only templates for building multiple Docker containers using layered Docker filesystems. The containers so created will share common files and enhance disk usage and downloads on the containers. Once the containers are created, multiple applications can be installed on them. The applications can also be updated to a new version by simply building a new layer on the existing images rather than replacing the whole image or entirely rebuilding the containers. The containers can be mapped to a single image or to multiple images. If a single image is alone over-utilized in creating the containers or if any image remains unmapped to the containers, then such unmapped images will remain under-utilized but still will occupy unnecessary disk space. If too many such unmapped images exists, then the creation of new images may be impacted and applications cannot be bundled to the existing images/containers resulting in a decreased reusability of disk space, increased disk usage and reduction in the building speed of the containers. This is why, administrators need to frequently check for the images and the disk space occupied by those images. The DockContImgTest test does this check.

This test reports the total number of images that are available on the Docker host/server. In addition, this test helps administrators to compare the numerical statistics of the images that are mapped to the containers and those that are not mapped to any container. Likewise, administrators can also be able to figure out the disk space utilization of images that are mapped to the containers and the disk space utilization of the images that are not mapped to the containers. Using this test, administrators can figure out the images that are sparsely utilized, the images that are consuming too much of disk space etc and take remedial actions to restrict the disk space utilization of the images.

Outputs of the test : One set of results for the target Docker server that is being monitored

Measurement	Description	Measurement Unit	Interpretation
total_images	Indicates the total number of images available in the Docker server.	Number
image_container	Indicates the number of images that are mapped to the Docker containers.	Number	Ideally, value of this measure should be high. A low value of this measure indicates that more number of images are left idle and more disk space is occupied by unused images. This may cause potential space crunch in the disk. The detailed diagnosis of this measure if enabled, provides details about the containers and the images that were mapped.
image_not_container	Indicates the number of images that are not mapped to any Docker container.	Number	Ideally, the value of this measure should be low. The detailed diagnosis of this measure if enabled, lists the images that are not mapped to any Docker containers.
size_img_container	Indicates the amount of disk space utilized by the images that are mapped to the Docker containers.	MB	The detailed diagnosis of this measure if enabled, lists the images and the containers.
size_image_not_container	Indicates the amount of disk space utilized by the images that were not mapped to the Docker containers.	MB	A high value of this measure indicates space crunch in the disk. The detailed diagnosis of this measure if enabled, lists the images, which were not mapped with containers.