CSS uses EVS and local disks to store your indexes. During cluster creation, you can specify the EVS disk type and specifications (the EVS disk size).
During cluster creation, a certain number of EVS disks can be attached to each node (corresponding to an ECS). You can calculate the total storage capacity of a CSS cluster based on the sizes of EVS disks attached to different ECSs. The EVS disk size is determined by the node specifications you selected when creating the cluster.