What is journald?

journald is the logging component of systemd, the modern init system used by many Linux distributions. It collects, stores, and forwards log messages from the kernel, system services, and applications, keeping them in a binary journal file that can be queried later.

Let's break it down

  • systemd: the first process that starts when Linux boots; it manages services and the overall system.
  • journald: a daemon (background program) that runs under systemd.
  • Log sources: kernel messages, systemd service output, user applications, and even remote logs.
  • Storage: logs are saved in a binary format under /var/log/journal (or /run/log/journal for temporary storage).
  • Access: you read the logs with the journalctl command, which can filter by time, service, priority, etc.

Why does it matter?

  • Centralized logging: All logs end up in one place, making troubleshooting easier.
  • Reliability: Binary storage prevents log loss during crashes and supports automatic rotation.
  • Powerful querying: journalctl lets you quickly find exactly the messages you need, using filters like service name, PID, or log level.
  • Security: Logs can be signed and stored with restricted permissions, reducing tampering.

Where is it used?

  • Most modern Linux distributions that use systemd, such as Ubuntu, Fedora, Debian, Arch Linux, and many cloud images.
  • Embedded devices and containers that rely on systemd for service management also use journald for their logs.
  • Any application that writes to stdout/stderr or uses the syslog API will have its output captured by journald automatically.

Good things about it

  • Unified view: No need to chase separate syslog files; everything is in one journal.
  • Fast and efficient: Binary format is compact and quick to write/read.
  • Rich metadata: Each log entry includes fields like timestamp, service name, PID, UID, and more.
  • Built‑in rotation and compression: Journald handles log file size limits automatically.
  • Remote forwarding: Can send logs to other systems or central log servers.

Not-so-good things

  • Binary format: Not human‑readable without journalctl, making manual inspection harder for beginners.
  • Disk usage: If not configured, the journal can grow large and consume significant space.
  • Compatibility: Some legacy tools expect plain text syslog files, so extra steps may be needed to export logs.
  • Learning curve: Understanding all journalctl options and configuration files can be overwhelming at first.