Figure 13-23 Control Group in Action
Union File System
The Union File System is a foundational building block
for a container. It is a file system service that was
developed for Linux to allow different file systems to be
layered on top of each other to create a combination, or
union, of the various files to create a single merged
representation of the contents. If you have ever worked
with photo editing software, this will probably make
sense to you. Say you want to remove someone or
something from a picture (like that ex you never speak
of). You take the base picture and then add layers on top
of it, and you add or remove pixels until you have a
whole new picture. If you want to remove layers you
created, you simply peel them back to expose the
previous image. Every layer you add on top takes
precedence over all the layers below it. In essence, this is
how a union file system works, and it is one of the main
reasons such a file system is so efficient at storage. Each
layer includes only what is new, and nothing is
duplicated from a previous layer. The layers are read-
only, which means they are locked in place. There is a
read/write layer that sits on top that you can use to
interact with the file system, but nothing you do will be
maintained unless you save your changes. If you want to
make your changes permanent, you have to put another
layer on top of the union file system. This is called a
copy-on-write operation. Figure 13-24 shows an example
of a container image and its layers.