QLC Flash

Overview

QLC (quad-level cell) flash stores four bits per NAND cell, occupying a deliberate middle ground between HDD and TLC SSD. HDD delivers good capacity per watt but poor bandwidth per TB; TLC SSD delivers plenty of performance but poor TB per watt and dollar per TB. QLC is the "happy medium" that suits read-heavy workloads — including AI training, which Meta observes is not especially write-heavy — where neither extreme is optimal. Meta's QLC deployment targets 32 MB/s per TB of application bandwidth, supported by 2x Sapphire Rapids CPUs with PCIe Gen5 x4 per drive across the 128TB and 150TB drive generations. The supplier NAND roadmap extends to 256TB, 512TB, and even 1PB drives; at those densities current server architectures hit limits — CPU power and network bandwidth become untenable, and thermal simulations show corner-case throttling at 256TB U.2 / 300TB DFM capacities, motivating the move to the E2 form factor for future deployments.