DirectMemory Modules (DMMs) are Storage Class Memory (SCM) drives used in FlashArrays to provide a high-speed read cache (DirectMemory Cache). In the FlashArray//XL architecture, these modules provide significant performance benefits for read-intensive workloads.
A fundamental configuration rule for DMMs on the //XL platform is that they cannot be installed in a DirectFlash Shelf (DFS) .
* DMMs must be installed directly into the controller chassis (the head unit) to leverage the lowest possible latency connection to the CPU and memory complex.
* The external backend fabric that connects to the shelves (even though it is NVMe-oF/RoCE) adds a minute amount of latency that negates the extreme speed advantage of SCM. Therefore, Purity only supports DMMs when they are local to the controllers.
* DMMs share the same physical form factor and slots as standard DirectFlash Modules (DFMs) in the chassis but are typically prioritized for specific slot ranges (e.g., the first few slots) for thermal and bus management.
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