U.2

SSD drives

All SSD drives operate on the principle of storing electrical charge in memory cells, which retain the data even after power is disconnected. These cells are grouped into blocks. The operating principle is similar across all SSDs, always defined by the technology of the memory cells, which primarily affect the lifespan and other properties of the drive. SSD drives are typically categorized by the bus used to connect them – most commonly SAS, SATA, or NVMe.

NVMe is currently the most widespread interface because it uses the multi-lane PCIe bus, enabling high read and write speeds. The maximum transfer speed is influenced by the bus, which allows us to roughly estimate the disk's speed. The bus speed depends on the PCIe generation and the number of lanes used – typically, 4 lanes are used, but there are also models with 8 and 2 lanes.

NVMe Drive Formats

When choosing an NVMe drive, it is important to consider the format – U.2, U.3, M.2, or EDSFF (E1, E3). Each format has specific physical implementation and typical usage in different systems with different interfaces, so it is essential to know the specifications of these formats and their compatibility before purchasing.

U.2

These are 2.5" drives primarily intended for servers. Compared to M.2, they are significantly larger, allowing for capacities up to several hundred TB. Servers often support hot-swap, meaning the drive can be replaced while the system is running. U.2 ports on backplanes either allow only NVMe drives, or there are hybrid ports that can also accept SATA, SATA-E, or SAS drives. Available U.2 drive thicknesses are 7 mm and 15 mm, so the choice depends on the server's space availability.

U.3

U.3 is the successor to U.2, offering the advantage of supporting multiple protocols on a single slot – NVMe, SAS, and SATA. Thanks to this support for the same protocols, U.3 drives maintain backward compatibility with U.2 infrastructure and preserve the same dimensions and connection options. This compatibility makes it possible to plug a U.3 drive into a U.2 backplane interface. Compared to U.2, it offers simpler connections and flexibility when replacing drives.

M.2

The M.2 format is becoming more popular in laptops, desktops, and server environments, mainly due to its small size and relatively low acquisition costs. It is a module that usually connects directly to the motherboard or through an additional PCIe card. When selecting a drive, it is important to pay attention to the physical key of the slot. A major advantage of M.2 is its support for several interfaces – SATA, PCI-e (2x/4x), USB, etc. "Keys" on M.2 drives serve as a prevention against inserting the drive or card into an incompatible interface. Interface options are described in the table below.
A disadvantage of this format is limited capacity (max. 8 TB) and lower lifespan compared to U.2/U.3, as well as the absence of Hot-Swap technology. Typically, the drive is used for booting the system or as a cache disk.

EDSFF

EDSFF (Enterprise and Datacenter SSD Form Factor) is a newer format designed for data center and server needs. It offers:
– better cooling,
– Hot-Swap support,
– and lower energy consumption.
All drives use the SFF-TA-1002 connector. The format is available in many form factors, resembling previous formats in size, but designed for high-end setups:

E1.S

The smallest in the EDSFF series, serving as a Hot-Swap alternative to M.2. It offers better cooling and larger capacities. It is available in 9.5 mm / 15 mm / 25 mm widths, which differ in passive cooler size and performance.

E1.L

A longer version of E1.S with the potential for even greater capacity. Suitable for energy-efficient servers requiring high performance and storage.

E3.S

Designed as a modern replacement for traditional 2.5” drives. While maintaining the same size, it offers more efficient cooling and higher power input. It is available in thicknesses of 7.5 mm and 16.8 mm (referred to as 2T) – the difference is again in the size of the passive cooler and power consumption.

E3.L

Similar to E1, this is a longer version of E3.S. It brings the benefits of the smaller model with the option for higher capacity and power input. It is also manufactured in two thicknesses (just like E3.S).