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U.2-15mm form factor

Definition

U.2 is an enterprise SSD form factor using a standardized 2.5-inch chassis with an SFF-8639 connector (PCIe / NVMe / SAS / SATA capable). U.2-15mm refers specifically to the 15mm-thick variant — the larger of the common U.2 z-heights (7mm / 15mm) — which accommodates more NAND packages per drive.

U.2-15mm is Meta's chosen form factor for its QLC flash tier in 2025.

Why U.2-15mm for QLC

Meta's 2025-03-04 QLC post:

"The Industry standard U.2-15mm is still a prevalent form factor across SSD suppliers and it enables us to potentially scale to 512TB capacity. E3 doesn't bring additional value over U.2 at the moment and the market adoption split between the 4 variants of E3 makes it less attractive. Pure Storage's DFMs can allow scaling up to 600TB with the same NAND package technology."

Three load-bearing properties:

  1. Physical volume — enough room to host the NAND-package count needed for large QLC drives (up to 512 TB standard, 600 TB with DFM).
  2. Industry standard — prevalent across SSD suppliers; no vendor lock-in.
  3. Dual-fit — a server designed for DFMs also accepts U.2 drives, enabling vendor diversity in a single rack design.

Why not the alternatives

  • E1.S — "not an ideal form factor to scale our QLC roadmap because its size limits the number of NAND packages per drive." Great for TLC at moderate densities, too small for QLC.
  • E3 — "E3 doesn't bring additional value over U.2 at the moment and the market adoption split between the 4 variants of E3 makes it less attractive." The 4-variant fragmentation is the killing blow for a hyperscaler that values supplier-substitutability.

Scale endpoints

  • Standard NVMe QLC SSDs in U.2-15mm: up to 512 TB per drive.
  • Pure Storage DFMs in U.2-15mm slots: up to 600 TB per drive.

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