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Understanding how new solid state drive technologies can benefit the data center

Richard Leonarz, Director of Memory Marketing, Samsung Business Division | May 27, 2016
By combining SSDs and HDDs in the right mix, performance gains are possible while keeping costs under control.

With V-NAND technology, cell towers are created by stacking multiple layers, with scaling shifted from 2D to 3D. Rather than making cells smaller as in 2D NAND, V-NAND features relaxed intercell dimensions while still achieving significantly higher capacities with stacking. The result is that V-NAND delivers improved performance and endurance over planar NAND.

The V-NAND technology also enables increased performance and endurance in data centers. Benefiting from a larger cell geometry lowers the Error Correction requirements seen in the smaller Planar NAND geometries. This means that V-NAND SSDs operate with less energy than traditional planar NAND SSD, and far less energy than HDDs with spinning motors. Faster V-NAND flash also allows SSDs to take full advantage of faster interfaces.

V-NAND based SSD’s are also deliver higher endurance due to the reduced ECC requirements and lower energy consumption.

Depending on the application, the benefits can range from more users able to access data on the same network, improved response times for data analytics, or increased drive-writes on the SSD storage space.

Improve speed and performance with PCIe and NVMe

While huge strides have been made to improve NAND structure for better endurance, capturing the full performance gains requires improvements to the software interface connecting the SSD to the computer.

Non-Volatile Memory Express (NVMe) and PCI Express (PCIe) SSD technologies are transforming the speed and performance of data centers. With the PCIe interface and NVMe protocol, storage subsystems deliver higher bandwidth, lower latency, and avoid performance bottlenecks, all of which drive high-caliber data center performance. The switch from SATA or SAS to a PCIe interface provides data centers substantially more bandwidth than was ever possible with the earlier generation SATA interface. PCIe SSDs can be connected directly to the CPU without Host Bus Adapters, further reducing latency.

In addition to the electrical interface, operating systems also need an improved software interface for higher storage performance. Historically, SSDs and HDDs used Advanced Host Controller Interface (AHCI), which posed a bottleneck effect for SSDs since it was originally designed for high latency HDDs on SATA interface. The AHCI stack adds additional translation layers and, based on its design, can only support one queue with up to 32 outstanding commands.  By the nature of the technology, SSDs are inherently capable of much higher transfer speeds at lower latencies, but without an optimized software interface they can not reach their full potential. 

The road there has been bumpy, and is finally starting to smooth out. Prior to any standard approach, SSD vendors incorporating PCIe interfaces had to write a proprietary driver for improved performance. NVMe emerged as a new specification, using a simplified, low latency stack between the application and SSD to reduce I/O overhead and provide higher performance and improved efficiency. Where AHCI supports one queue and 32 commands, NVMe includes a vastly improved queueing system with support for thousands of queues, each allowing for up to 65,536 outstanding commands.  The transition to NVMe and PCIe SSDs contributes to improved random and sequential performance compared to SATA interface SSDs using the AHCI protocol.

 

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