K8S Notes

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Storage

• Ephemeral Nature of Containers - Storage Perspective:
  • Containers have a notable limitation from a storage perspective: they are ephemeral.
  • Ephemeral implies temporary or short-lived, highlighting a key characteristic of containers.
• Container Image Structure:
  • Container images are primarily read-only.
  • Comprise different layers, each capturing additions made during the build phase.
  • This structure ensures consistent behavior and functionality when starting containers from the image.
• Read-Write Layer for File Writing:
  • Many applications require writing files during execution.
  • To facilitate file writing, a read-write layer is added on top of the container image when a container is started from an image.
  • This read-write layer allows the temporary modification of the container’s file system during its runtime. Container Layers, retrieved from the Docker documentation
• Challenge of Data Persistence in Containers:
  • The read-write layer in containers is ephemeral; it is lost when the container is stopped or deleted.
  • Similar to computer memory erasure when shut down.
• Persistence Through Writing to Disk:
  • To retain data, it must be written to disk before container shutdown or deletion.
• Using Volumes for Persistent Data:
  • Containers can use volumes to achieve data persistence.
  • Concept: Instead of isolating the entire filesystem, specific directories on the host are passed into the container filesystem.
  • While this ensures data persistence, it weakens the isolation of the container as it effectively grants access to the host filesystem. Data is shared between two containers on the same host
• Challenges in Container Orchestration for Data Persistence:
  • Orchestrating numerous containers introduces challenges beyond persisting data on the host where the container originated.
  • Common scenarios include data access by multiple containers on different host systems or when a container starts on a different host but still requires access to its volume.
• Role of Container Orchestration Systems (e.g., Kubernetes):
  • Container orchestration systems, like Kubernetes, can address these challenges.
  • Effective resolution, however, relies on a robust storage system connected to the host servers.
  • The orchestration system facilitates coordination and management, but a reliable storage infrastructure is crucial for seamless data access and sharing across containers and hosts. Storage is provisioned via a central storage system. Containers on Server A and Server B can share a volume to read and write data
• Standardization for Diverse Storage Implementations:
  • The continuous expansion of various storage solutions prompted the need for a standardized approach.
  • The Container Storage Interface (CSI) was introduced to provide a uniform interface, enabling the attachment of different storage systems.
  • This standardization facilitates seamless integration of diverse storage solutions, whether they are cloud-based or on-premises.

Container Orchestration