So far, we only worked with workloads that we could start and stop at will, with no issue. However, real-world applications often carry state and record data that we prefer (even insist) not to lose. The transient nature of containers themselves can be a big challenge. If you recall our discussion of layered filesystems in Chapter 1, Introduction to Kubernetes, the top layer is writable. (It's also frosting, which is delicious.) However, when the container dies, the data goes with it. The same is true for crashed containers that Kubernetes restarts.
This is where volumes or disks come into play. Volumes exist outside the container and are coupled to the pod, which allows us to save our important data across containers outages. Further more, if we have a volume at the pod level, data can be shared between containers in the same application stack and within the same pod. A volume itself on Kubernetes is a directory, which the Pod provides to the containers running on it....