db-io-reduce

Database service containers and ad-hoc docker run commands in CI write to disk by default. On GitHub Actions hosted runners, this causes unnecessary I/O overhead that can significantly slow down test suites.

Optimise by either:

1. Using --tmpfs to mount the database data directory as an in-memory filesystem.
2. Configuring the database to reduce disk I/O (disk flushes are a major source of I/O overhead on CI runners):
   • MySQL: Set innodb_flush_log_at_trx_commit=2.
   • PostgreSQL: Set fsync=off (acceptable for ephemeral CI environments).

Scope

This rule checks three patterns:

• Service containers: jobs.<id>.services.*.image matching MySQL or PostgreSQL.
• Direct docker run: Steps that call docker run with a MySQL or PostgreSQL image.
• docker compose: Steps that call docker compose up/start/run when the step name or command mentions MySQL/PostgreSQL.

Examples

Not optimal

services:
  mysql:
    image: mysql:8
    env:
      MYSQL_ALLOW_EMPTY_PASSWORD: yes

steps:
  - run: docker run -d -p 3306:3306 mysql:8

Optimal (tmpfs)

services:
  mysql:
    image: mysql:8
    options: --tmpfs /var/lib/mysql
    env:
      MYSQL_ALLOW_EMPTY_PASSWORD: yes

Optimal (config flag)

services:
  postgres:
    image: postgres:14
    env:
      POSTGRES_PASSWORD: password
      PGOPTIONS: "-c fsync=off"