E3.S vs U.2: Which Server SSD Form Factor Should You Use?

E3.S and U.2 are two NVMe SSD form factors for servers and storage systems. U.2 is the 2.5-inch drive shape that has been the data center standard for about a decade. E3.S is the newer EDSFF (Enterprise and Data Center Standard Form Factor) drive, designed for PCIe Gen5 and beyond, with better cooling, more power headroom and more drives per server.

Both are hot-swappable NVMe drives, but they are not interchangeable: each needs its own slot, connector and drive bay. The right choice usually depends on the server platform you are buying or supporting.

What Is an E3.S SSD?

An E3.S SSD is a hot-swappable NVMe drive in the EDSFF E3 Short form factor, defined by SNIA in SFF-TA-1008. It measures 76mm × 112.75mm and comes in two thicknesses: E3.S (1T) at 7.5mm, rated up to 25W, and E3.S 2T at 16.8mm, rated up to 40W. It uses the EDSFF card-edge connector (SFF-TA-1002), which supports x4, x8 and x16 PCIe links.

E3.S was designed for PCIe Gen5 and later generations, and the same E3 slots can also hold other devices, such as CXL memory expansion modules. See AMP’s E3.S SSDs.

What Is a U.2 SSD?

A U.2 SSD is an NVMe drive in the familiar 2.5-inch hard-drive shape, usually 15mm thick, using the SFF-8639 connector with four PCIe lanes. It became the data center standard because it fits the same 2.5-inch bays used for SAS and SATA drives. U.2 drives are available from PCIe Gen3 through Gen5.

U.3 uses the same 2.5-inch shape and connector but adds tri-mode support (SFF-TA-1001), so one bay can accept SAS, SATA or NVMe drives through a tri-mode controller. See AMP’s U.2 and U.3 SSDs.

E3.S vs U.2 at a Glance

Feature	E3.S	U.2 / U.3
Form factor family	EDSFF (SNIA SFF-TA-1008)	2.5-inch (SFF-8639; U.3 adds SFF-TA-1001)
Size	76 × 112.75mm; 7.5mm (1T) or 16.8mm (2T)	69.85 × 100.45mm; usually 15mm
Connector	EDSFF card edge (SFF-TA-1002)	SFF-8639
PCIe lanes	x4, x8 or x16	x4 (or x2 + x2 dual-port)
Power	Up to 25W (1T) or 40W (2T)	Typically up to about 25W
Designed for	PCIe Gen5 and later	PCIe Gen3 to Gen5
Hot-swap	Yes	Yes
Other devices in the slot	CXL memory and accelerators	Drives only
Best fit	New Gen5 servers with EDSFF bays	Existing and current 2.5-inch NVMe systems

Key Differences Between E3.S and U.2

Density

E3.S drives are thinner than 15mm U.2 drives and stand vertically in the front of the server, so a chassis can fit more drives across the same front panel. That means more capacity per rack unit.

Cooling and power

U.2 was designed around hard-drive bays with limited airflow. E3.S has a flat, open shape that lets air pass over it more easily, and its specification allows up to 25W (1T) or 40W (2T). That headroom matters because PCIe Gen5 drives run hotter and draw more power than Gen4 drives.

Performance

At the same PCIe generation and lane count, an E3.S drive and a U.2 drive can reach similar speeds; the interface sets the limit, not the shape. E3.S adds room to grow, with support for x8 and x16 links and the power to sustain Gen5 and future Gen6 speeds without throttling.

Compatibility

U.2 fits the huge installed base of 2.5-inch bays, and U.3 adds tri-mode flexibility. E3.S needs servers designed with EDSFF bays. Adapters exist, but they are a workaround, not a substitute for native support.

Future platforms

Many new PCIe Gen5 server and storage platforms ship with E3.S bays, and the E3 slot is shared with CXL memory and other devices. For new designs, E3.S is the long-term direction; U.2 will stay common for years in existing systems.

Where Does E1.S Fit?

E1.S is the smaller EDSFF drive, about 31.5mm wide, built for 1U servers and compact, dense systems such as hyperscale and edge servers. E1.S fits more drives into very tight spaces; E3.S fits more capacity and power per drive. See AMP’s E1.S SSDs and why EDSFF SSDs suit data centers.

Which Should You Choose?

Choose U.2 or U.3 if:

• Your servers or storage systems have 2.5-inch NVMe bays
• You are maintaining or expanding an existing platform
• You need tri-mode bays that also accept SAS or SATA drives (U.3)

Choose E3.S if:

• You are deploying new PCIe Gen5 servers built with EDSFF bays
• You need more drives and capacity per rack unit
• Your drives will run at high power, where cooling and power headroom matter
• You want a platform ready for CXL devices and future PCIe generations

Moving from U.2 to E3.S

Switching form factors usually happens with a new server generation, since E3.S needs EDSFF bays. Before you move, confirm:

• Which E3.S thickness your chassis supports: 1T (7.5mm) or 2T (16.8mm)
• The PCIe generation and lane width of each bay
• Single-port or dual-port requirements for failover
• Capacity and endurance per drive, read-intensive (1 DWPD) or mixed-use (3 DWPD)
• Power and airflow limits of the server

E3.S and U.2 SSDs from AMP

Accelerated Memory Production, Inc. (AMP) supplies both form factors. AMP E3.S SSDs run on PCIe Gen5 x4 at up to 14,800 MB/s, from 1.6TB to 15.36TB, in single-port and dual-port versions. AMP U.2 and U.3 SSDs cover enterprise and industrial NVMe needs for 2.5-inch systems.

For help choosing between them, or matching a drive to your server, contact AMP at 714-460-9800 or send us a message online.

FAQs About E3.S and U.2