9.7.1.1 Calculating the Physical Size for Grid Disks

You can use the following formula to get a rough estimate of the total physical space to set aside for a sparse ASM disk group:

Total physical space =
   (SUM(size of all test masters in the sparse ASM disk group) +
    SUM(approximate size of all updates to the snapshot databases))
   * ASM Redundancy

In the formula above, ASM redundancy takes into account ASM mirroring of extents. Exadata requires ASM redundancy set to either normal redundancy (double mirror the extents) or high redundancy (triple mirror the extents). If the sparse ASM disk group will use normal redundancy, expect to double the space used. If using high redundancy, expect to triple the space used.

For example, if you want 2 test masters in the sparse ASM disk group created with normal redundancy with a combined total space of 500 GB (250 GB each) and each test master will have 5 Exadata snapshots with the expectation that each snapshot will modify 20% of the blocks, then the total physical space that will be needed can be calculated as follows:

Space for 2 test masters:   2 * 250 GB =                                500 GB
Space for 5 snapshots per test master, for a total of 10 snapshots:
    10 * 250 GB * 20% =                                                 500 GB
Subtotal                                                               1000 GB
Normal redundancy: 2 * 1000 GB =                                       2000 GB

Divide this value by the number of disks to determine the size parameter for each disk. ASM grid disks should be allocated on 16 MB boundaries. If the size parameter in MB for each grid disk is not evenly divisible by 16, adjust up to a 16 MB boundary.

Note that you should set aside additional space to use for multiple projects and through multiple iterations.

Also to accommodate any disk rebalancing operations, you should add a 15% space cushion on top of the space used for snapshots and test masters.