END probes usually do not fire until tracing has been explicitly stopped by the DTrace consumer. END probes are guaranteed to fire only on one CPU, but the CPU on which the probe fires is undefined. With fill buffers, tracing is explicitly stopped when at least one of the per-CPU principal buffers has been marked as filled. If the fill policy is selected, the END probe might fire on a CPU that has a filled buffer. To accommodate END tracing in fill buffers, DTrace calculates the amount of space that is potentially consumed by END probes and subtracts this space from the size of the principal buffer. If the net size is negative, DTrace does not start and dtrace outputs the following error message:

dtrace: END enablings exceed size of principal buffer

The reservation mechanism ensures that a full buffer always has sufficient space for any END probes.