## Planning a Volume Snapshot

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:

- Snapshot reserve space is consumed each time a block is written to the primary volume that was not written since the snapshot was created or when a block is written to the snapshot volume for the first time. Estimating the rate at which these activities occur can be difficult.
- Some space is reserved for the snapshot metadata, which requires 7% of the size of the primary volume. For example, for a primary volume of 100 GB in which every block will change, each snapshot requires 107 GB. If you specify that eight snapshots are created, the system allocates 856 GB.
- After you specify a volume's snapshot reserve space, you cannot modify its size. If you do not allocate enough snapshot reserve space and the space is filled, the subsequent write operations to the primary volume are completed without copying the changed data to the snapshot reserve space. Also, any read or write operations to the snapshot volume are rejected. The snapshot is no longer valid. To prevent running out of space, you must delete existing snapshots as required to make space for new ones.

### Related Topics

- About Volume Snapshots
- Creating a Volume Snapshot
- Deleting a Volume Snapshot