Calculating usable disk capacity
Determining how much data your Cassandra nodes can hold.
To calculate how much data your Cassandra nodes can hold, calculate the usable disk capacity per node and then multiply that by the number of nodes in your cluster. Remember that in a production cluster, you will typically have your commit log and data directories on different disks.
Start with the raw capacity of the physical disks:
raw_capacity = disk_size * number_of_data_disks
Calculate the usable disk space as follows:
( raw_capacity * 0.9 ) = formatted_disk_spaceDuring normal operations, Cassandra routinely requires disk capacity for compaction and repair operations. For optimal performance and cluster health, DataStax recommends not filling your disks to capacity, but running at 50% to 80% capacity depending on the compaction strategy and size of the compactions.
Account for file system formatting overhead (roughly 10 percent):
formatted_disk_space * (0.5 to 0.8) = usable_disk_space