Why would I change this percentage?

Reserved capacity is typically 20 percent of the base volume for asynchronous mirroring operations. Usually this capacity is sufficient.

The capacity needed varies, depending on the frequency and size of I/O writes to the base volume and how long you intend to use the storage object's copy service operation. In general, choose a larger percentage for reserved capacity if one or both of these conditions exist:

• If the lifespan of a particular storage object's copy service operation will be very long.
• If a large percentage of data blocks will change on the base volume due to heavy I/O activity. Use historical performance data or other operating system utilities to help you determine typical I/O activity to the base volume.