FlashArray performance and capacity efficiency are optimized when drives are grouped into "Wide Write Groups" (typically 20 modules). Purity OS generally enforces strict homogeneity within a write group to ensure consistent performance and reliability. Specifically, mixing standard DirectFlash Modules (DFM) with DirectFlash Modules with Distributed NVRAM (DFMD) in the same write group is restricted by default because of the architectural differences in how they handle persistent memory and I/O acknowledgment.
However, in upgrade scenarios where a customer is expanding an existing 16-module DFM group to a 20- module group using newer DFMD spares or expansion packs, Purity includes an engineering tunable to permit this mixed configuration. The correct procedure is to physically install the new DFMDs and run the admission command. Because of the mix, the array might initially hesitate or create a separate group. To force the creation of a single 20-drive Wide Write Group (WWG) , the Implementation Engineer must work with Pure Storage Support to apply the specific tunable puretune --set ALLOW_DFMD_IN_DFM_WWG 1 .
This overrides the default restriction, allowing the system to merge the disparate module types into a single, high-efficiency capacity pool.
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