Flash Storage Interface Technologies and Development Trends

As the final article in this series, we discuss flash memory interface technologies and their development trends. SSDs, with superior random performance, increasing capacities, and decreasing costs, are poised to replace HDDs, and their designs inherit some HDD interface concepts.

Most SSDs today use SATA/SAS interfaces, which were originally designed for HDDs; the massive speed increase of flash makes interface bandwidth the primary I/O bottleneck. Consequently, PCIe‑based SSDs have become widespread to meet high‑performance demands, and Micron is exploring serial NOR flash interfaces. To date, many storage vendors have released PCIe flash cards (e.g., EMC XtremSF PCIe SSD card) and disks.

Fusion‑io’s patented PCIe flash card (IO‑Drive) can be added to servers to accelerate applications with microsecond‑level latency, a performance level unattainable with SATA/SAS, indicating an inevitable major shift in flash interface standards.

The NVMe standard was proposed for PCIe interfaces (though NVMe is not limited to PCIe). Thirteen founding companies—including IDT, Dell, Intel, EMC, NetApp, Oracle, and Cisco—along with over 80 industry leaders, developed this scalable host controller interface for PCIe SSDs, defining optimized register interfaces, instruction sets, and functional sets to unlock current and future SSD performance potential. This reduces the need for OEMs to standardize multiple SSD drivers, accelerating NVMe SSD adoption. NVMe also supports the U.2 (SFF‑8639) interface, which includes four PCIe lanes and two SAS/SATA lanes, as well as SATA Express and M.2, all claiming NVMe compatibility. While not all PCIe SSDs implement NVMe, a PCIe SSD that follows the NVMe spec must comply with its requirements.

OCZ, a storage solutions provider under Toshiba, introduced the NVMe‑based Z‑Drive 6000 series SSD, combining third‑generation PCIe with the NVMe command set and queue design to deliver rapid access to critical data and high resilience for system integrators and storage vendors.

OCZ offers three Z‑Drive 6000 models: the 2.5‑inch SFF series with up to 3.2 TB capacity, the lower‑performance but higher‑capacity 6.4 TB Z‑Drive 6300 SFF series, and the half‑height, half‑length AIC series (Z‑Drive 6300 AIC) also up to 6.4 TB.

Traditional NOR flash uses an SRAM interface (more pins than parallel NAND, supports XIP code execution, but offers limited capacity) and allows arbitrary byte writes at any address.

Micron’s XTREMFlash serial NOR flash solution delivers up to 3.2 Gb/s read/write performance and is compatible with widely used serial NOR interfaces, surpassing all flash storage types except massive NAND arrays.

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