It is important to understand that usable capacity should always be less than raw capacity, while effective capacity, applicable in the context of data reduction technologies, is typically greater. The relationship can be expressed as Effective Capacity = Usable Capacity multiplied by the Data Reduction Factor.
RAW Space represents the total disk space accessible to the array subsystem. For example, an EVA configured with eight 300GB disks would provide a total of 2.4TB of RAW space. After accounting for Single Disk protection, the resulting Usable Space would be 1.8TB.
The term “usable ratio” describes the relationship between usable capacity and raw capacity, defined as usable capacity divided by raw capacity, which is always equal to or greater than one. To determine the total raw capacity of a storage system, one must multiply the raw capacity of each drive-set feature by its respective quantity and then aggregate these values. A straightforward approach for calculating RAID 5 capacity involves taking the disk drive’s capacity, such as 146 GB, and reducing it by approximately 15% to estimate the usable capacity available to hosts.
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