What is a difference between compression and deduplication within a VxRail OSA cluster?

Within a VxRail cluster operating on the Original Storage Architecture (OSA), optimizing storage capacity is driven by software-defined data efficiency policies. Starting with vSphere 7.0 Update 1, vSAN introduced the option to enable "Compression Only" as a distinct toggle from the combined "Deduplication and Compression" feature set. A fundamental architectural distinction between these two mechanisms lies in their operational scope and data reduction boundaries.

When the compression-only feature is active, data reduction algorithms execute strictly on a per-disk basis. This means blocks are evaluated and compressed locally before being written directly to that specific capacity drive, avoiding structural dependencies on adjacent drives or affecting the broader disk group cache-to-capacity relationship. In contrast, deduplication inherently requires a cross-device scope because it eliminates redundant data blocks across a larger logical boundary; hence, deduplication is applied to the entire disk group. For deduplication to occur, the hashing algorithms must track data signatures across all capacity drives tied to a specific caching drive within that given disk group. Consequently, deduplication carries a wider failure domain and distinct architectural requirements compared to localized per-disk compression profiles.