Checkpoint flushes all buffers to disk and deletes xlogs
checkpoint_timeout: Maximum allowed time since the last checkpoint. checkpoint_completion_target: Target length of the checkpoint. Instead of using all possible I/O in a short time it is spread across the target value to avoid high spikes in I/O, the value is checkpoint_completion_target * checkpoint_timeout checkpoint_segments: When xlogs hit this qty a checkpoint is triggered.
Most CPs should be due to timeout.