You have the option of allocating the snapshot reserve space for a volume when you create the volume, or you can allocate the snapshot reserve space later, using the Volume Details page. The advantage to allocating the snapshot reserve space at the time you create the volume is that you can be assured that there is sufficient free space for the snapshots. When you allocate the snapshot reserve space at a later time, the amount of required free space might not be available.
The New Snapshot wizard guides you through the process of allocating the snapshot reserve space. To eliminate the risk of running out of space, always set the snapshot reserve space to the 100% level of activity, indicating that every block will change, for the following reasons: