NVMe and AHCI

http://elf8848.iteye.com/blog/1731274

AHCI:NCQ technology, 600MB/S, one queue, 32 instructions per queue

nvme:65000 queues, 65000 instructions per queue, 3.2gb/s

What is NVMe?

Thenon-volatile memory Express non-volatile storage standard, the NVME specification, was formally released in 2011, specifically for NVND Flash and next-generation storage devices. It is developed based on the characteristics of flash memory and is designed to minimize the gap between storage systems and memory bandwidth. There are many products on enterprise SSDs that use the NVME specification, but consumer-grade products are much slower, and the latest standard is NVMe 1.2.

The benefits of NVMe?

(1) Lower delay:

The three major factors that cause the latency of the hard disk storage, the storage media itself, the controller and the software interface standards. It can be found that the delay between the controller and the Software interface section can be significantly reduced compared to AHCI,NVME for the following two reasons:

NVMe is geared toward a PCIe SSD, where the native PCIe master is directly connected to the CPU, not the traditional way, through the South Bridge controller, and then to the CPU (of course, if the device is plugged into the PCIe interface provided by the South Bridge, then this part of the delay will not be much lower);

NVMe streamlines the invocation by eliminating the need to read registers when executing commands, while the AHCI command reads 4 registers and consumes 8,000 CPU cycles, resulting in 2.5μs delays.

(2) Higher transmission performance

Another focus of NVME is to increase the performance of SSD ioPS (read and write times per second). If we have a detailed understanding of the SSD's master parameters, we should know that there will be a queue depth this parameter. Theoretically,iops= queue depth/IO latency , so IOPS performance, and queue depth has a greater relationship (but IOPS is not proportional to the queue depth, because in real-world applications, with the increase in queue depth, the IO latency will increase). The best-performing SATA interface SSD on the market can reach 32 in the queue depth, but this is the limit that AHCI can do. But today's high-end enterprise-Class PCIe SSDs, with queue depths of up to 128 or even 256, can deliver the highest IOPS performance. In the NVMe standard, the maximum queue depth is up to 64000. In addition, the number of NVMe queues has increased by 64000 from the 1 of AHCI.

(3) Lower power consumption control

Lower Energy Management

NVMe joins the automatic power state switching and dynamic energy management functions, as shown in the example, the device from power consumption status 0 idle 50ms can quickly switch to energy State 1, after 500ms idle will enter a lower energy consumption state 2. While switching the power state creates a short delay, the power dissipation in these two states can be controlled at very low levels when idle, so there is a greater advantage in energy management than the mainstream SATA interface SSD, which is especially helpful for increasing the battery life of mobile devices such as laptops. In addition, since the native PCIe can be directly connected to the CPU, the data transfer without the power generated during the intermediate transfer process will also reduce energy consumption to some extent.

NVMe and AHCI