HP StorageWorks 2000 Series Reference Manual page 110

Each snap pool has three policy levels that notify you when the snap pool is
reaching decreasing capacity. Each policy level has an associated policy that
specifies system behavior when an associated threshold value is reached. The
following table summarizes the default thresholds and policies. You can set the
Warning and Error thresholds and the Error and Critical policies.
Warning 75%
Error 90%
Critical 99%
SMU notifies you of events based on your event notification settings. See
"Configuring Event Notification" on page 53 for more information about how and
under what conditions the system alerts you when specific events occur.
Policy Trigger Behavior
A policy might be triggered before it appears that a specified threshold has been
reached. This is because a threshold percentage is based on the size of the snap
pool, less a fixed amount of 750 Mbyte for internal use. This fixed amount
guarantees that there is enough reserve space to store pending data for which the
controller has space. The following example demonstrates policy trigger behavior:
Snap pool size = 10,000 Mbyte (10 Gbyte)
Snap pool reserve = 750 Mbyte
Space available = 9,250 Mbyte
Policy trigger set at default error level of 90% = 9,250 Mbyte x 0.9 = 8,325
Mbyte
In the above example, the 90% trigger occurs at 8.325 Gbyte. If you did not take
into account the amount of reserve space, you might expect the trigger to occur at 9
Gbyte (10 Gbyte x 0.9). For larger snap pools, the impact of this reserve space is
less noticeable.
Notify Only.
Delete Oldest Snapshots.
Delete Snapshots.