Recording modes

Continuous

Every frame from the camera is written to disk the whole time it's active. This is the well-understood default, and the recommended starting point for a new camera while its motion detector hasn't been tuned to its specific scene yet.

Motion

Motion-mode cameras buffer segments in a RAM cache (tmpfs, sized by MOTION_CACHE_TMPFS_BYTES) and persist to disk only when motion is actually detected: the buffered pre-roll leading up to the event, the event itself, and a configured post-roll afterward. Idle time between events never touches disk at all, that's the entire point of the mode.

Two safety rails make this reasonable to leave running unattended:

• Fail-open. The instant a camera's motion detector becomes unhealthy, a stalled sub-stream, a dead decoder, anything that means Crumb can no longer form a trustworthy keep/discard decision, that camera immediately starts persisting everything to disk, exactly like Continuous mode, until detection is verified healthy again. A health alert fires for the duration. "I can't tell if this is interesting" always resolves to "record it all," never to "record nothing."
• Spill. If the RAM cache nears its configured size, because of many cameras, a burst of concurrent motion, or a slow disk, the oldest buffered segments are persisted to disk rather than being dropped from the cache unwritten. Cache pressure can change when something gets written, never whether it survives.

Recommendation for a new camera

Start a new camera on Continuous, or on Motion with shadow mode enabled (records everything as normal, but stamps each segment with the keep/discard verdict the motion buffer would have made), until its motion detector is tuned against real footage from that camera's actual scene. Only switch a camera to live Motion mode after checking what would have been discarded under shadow mode. An untuned detector missing a real event is a worse outcome than the disk space Motion mode saves.

See Motion & Detection for tuning the detector itself.