The capacity utilization for storage is one area where storage vendors have made a lot of improvements. Advanced features such as storage pooling, thin provisioning, and storage virtualization have introduced greater efficiencies for using storage capacity.
Still, trying to understand capacity utilization can be confusing. The utilization must be examined at a larger scale than a single storage system. Storage virtualization can span systems. Thin provisioning overcommits capacity across systems with the ability to drive up utilization rates. The larger the pool, the more flexibility is allowed for a system in allocating storage resources.
Data reduction (compression and/or deduplication) usually allows more data to be stored in a given amount of storage. Data reduction effectiveness varies based on the data type and the implementation by the vendor. Data reduction represents a potential increase in usable capacity. Guidelines or guarantees from the vendor can be used to gauge that potential, and actual measurements are usually available from the management interfaces on the storage systems when data reduction is in use.
In the discussion about storage capacity utilization, it is useful understand basic definitions and update them to current terminology for the technology in use.
Used capacity – where the data is stored that can be accessed from hosts.
Usable capacity –storage space within a storage system or across pooled systems that can be configured for volumes (LUNs) or filesystems. This is the capacity minus the storage system overhead. The overhead includes data protection such as RAID devices and allocated chunks in storage pools and segments for forward error correction using correcting codes such aserasure codes. Filesystems also reserve space for operational processes, which is not included in the usable capacity calculation.