How images are stored on disk.
WHAT A STORAGE DRIVER DOES
Combines the read-only layers and the writable layer into one filesystem view.
WHAT THAT MECHANISM IS CALLED
A union filesystem.
WHAT HAPPENS ON WRITE
The file is copied up into the writable layer first.
WHAT THAT MEANS
Writing to a large file inside a container is slow the first time.
WHY THAT MATTERS
Databases and anything write-heavy perform poorly on the container filesystem.
WHAT TO DO
Put write-heavy data on a volume, which bypasses the union layer.
WHAT LAYER SHARING PROVIDES
Images built on the same base share those layers on disk and in transfer.
WHY THAT ARGUES FOR CONSISTENCY
Using one base image across your services saves substantial space.
WHAT COUNTS AS A LAYER
Each instruction that changes the filesystem.
WHY FEWER IS BETTER, WITHIN REASON
Each has overhead, and deleted content in earlier layers persists.
WHAT TO BALANCE AGAINST THAT
Cache efficiency, which favours separate layers for things that change at different rates.
WHAT TO CHECK WHEN IMAGES ARE UNEXPECTEDLY LARGE
The size contributed by each layer.
WHAT USUALLY EXPLAINS IT
Package caches Build artefacts Files deleted in a later layer
WHAT TO DO
Combine install and cleanup into one instruction, or use a multi-stage build.
