Swap

Swap is disk space the kernel uses as extra memory. It delays out-of-memory kills, but memory on disk is hundreds of times slower, so every service on the server slows down.

How it works

With --memory set and no --memory-swap, a container may also use as much swap as memory. Setting --memory-swap equal to --memory forbids swap for that container: it is killed at its limit instead of dragging the server down.

What breaks

A server that swaps shows no crash, only latency climbing for every service: timeouts, failing healthchecks, slow databases. docker stats and free -m make it visible; the fix is a limit on the service that grows.

In a terminal

docker run -d --name api --memory 256m --memory-swap 256m orders-api
free -m

See it break, then fix it

Write the Dockerfile or the compose file, type the docker commands, and watch a simulated server run them through reboots, upgrades and traffic. Free to start.

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