World's first hybrid CXL device combines flash memory and DRAM — storage tiering comes to remote memory over PCIe
Samsung has unveiled a new Compute Express Link (CXL) Add-in Card called the CXL Memory Module-Hybrid for Tiered Memory (CMM-H TM), which adds additional RAM and flash memory that can be remotely accessed by CPUs and accelerators. The expansion card comes with a mixture of high-speed DRAM and NAND flash and is intended to provide a cost-effective way to boost memory capacity for servers without using locally installed DDR5 memory, which often isn't an option in oversubscribed servers.
Samsung's solution runs on the Compute Express Link (CXL), an open industry standard that provides a cache-coherent interconnect between CPUs and accelerators, thus allowing CPUs to use the same memory regions as connected devices utilizing CXL. The remote memory, or in this case, a hybrid RAM/flash memory device, is accessible over the PCIe bus, which comes at the cost of ~170-250ns of latency, or roughly the cost of a NUMA hop. CXL was introduced in 2019 and is in its third revision, featuring PCIe 6.0 support.
The CXL spec supports three types of devices: Type 1 devices are accelerators that lack local memory, Type 2 devices are accelerators with their own memory (like GPUs, FPGAs, and ASICs with DDR or HBM), and Type 3 devices consist of memory devices. The Samsung device falls into the Type 3 category.
