Simple definition
Conserve Mode is a FortiOS protection state triggered when available memory reaches critical thresholds.
Technical definition
FortiOS monitors memory usage and can enter conserve mode to preserve the resources required to keep the system operating. Depending on the situation and enabled features, new sessions or selected processing can be restricted until memory pressure falls. Exact thresholds and behavior can vary by FortiOS version and platform.
How it works / role
Its role is to prevent severe memory pressure from causing a more serious FortiGate failure. Conserve mode is a symptom that must be investigated, not the root cause by itself.
What is it used for?
Keep the firewall operational as far as possible when a process, session volume, or inspection feature consumes excessive memory.
Practical example
A FortiGate experiences a sharp rise in sessions and memory usage. FortiOS enters conserve mode and limits selected new processing until usage returns below recovery thresholds.
Common issues
- Restarting the device without identifying what consumed the memory
- Abnormally high session volume
- Process or inspection feature consumes excessive memory
- Platform capacity is insufficient for the real workload
Related terms
Key takeaway: When a FortiGate enters conserve mode, correlate the event with memory usage, processes, sessions, and traffic load to identify the actual cause.