Shulou(Shulou.com)06/02 Report--

[IT168 compilation] NVMe, short for Non-Volatile Memory express (non-volatile memory Host Controller Interface Specification), is a protocol that enables solid state drives (SSD) to run faster, and is becoming more and more popular among enterprise users.

The easiest way to understand what a NVMe is is an analogy-suppose you just bought a sports car with a speed of 400km / h, three or four times the speed of your old car. The only problem is that ordinary roads are not allowed to drive at this speed, and the general urban road speed limit of dozens of kilometers per hour, often have to consider the problem of traffic flow. If you want to make the sports car faster, you need to drive in a different way.

This scenario is a bit like what happened in the storage industry after the manufacturer launched SSD based on flash technology. Flash memory technology is many times faster than traditional mechanical hard drives (depending on the device you use), but early devices used the same SATA or SAS as hard drives to connect to the system and network. Although these interfaces are sufficient for the performance that hard disk drives (HDD) can provide, they pose a bottleneck for SSD. This is a particular problem for enterprise big data analytics and other data-intensive workloads.

These performance issues have prompted people to look for better ways to connect SSD to the server, which is an opportunity for NVMe to use its talents.

1. What is NVMe?

The NVMe protocol standard is overseen by NVM Express, an alliance of more than 100 organizations dedicated to developing faster protocols to improve the performance of non-volatile storage. The organization is headed by a board of directors of 13 companies, including Cavium, Cisco, Dell EMC, Facebook, Intel, Micron, Microsemi, Microsoft, NetApp, Samsung, Seagate, Toshiba memory and Western Digital.

The maximum theoretical speed that can be achieved by HDD is compared with that provided by SATA and NVMe.

The official definition of NVMe describes it as "an open collection of standards and information to fully unlock the advantages that non-volatile storage can offer in all types of computing environments, from mobile to data centers. NVMe is designed from the bottom to provide high-bandwidth and low-latency storage access for current and future NVM technologies."

NVMe is an interface specification that connects storage to the server through the PCI Express bus. In short, it makes SSD communicate with the host system faster. It helps alleviate bottlenecks when flash is connected to the system through SAS or SATA originally designed for HDD.

2. NVMe is much faster than SATA or SAS

NVMe controllers improve performance in several different ways. One is to use the PCIe bus, which connects storage directly to the system CPU. This direct connection eliminates some of the necessary steps for SATA and improves overall performance.

In addition, NVMe SSD achieves parallelism to a large extent, greatly improving throughput. When data is transferred from storage to the server host, it enters a row or queue. Traditional SATA connections can only support one queue and can only receive 32 data at a time. Going back to the car analogy, it's like there's only one lane of traffic that can hold 32 cars.

NVMe storage supports up to 64000 queues, each with 64000 entries. In other words, it's like walking from an one-way street to a 64000-lane road, each of which can hold 64000 cars, which has a huge impact on overall performance.

Just as our analogy says that the speed limit for urban roads is generally tens of kilometers per hour, there is also a speed limit for SATA and SAS connections. For SATA, the theoretical maximum transmission speed is 6.0Gbps (in practice, the maximum transmission speed is much lower). This is equivalent to setting an upper limit on the speed of SATA SSD. Beyond a certain limit, the use of fast flash memory has no impact on the overall performance of the system, because SATA connections can cause bottlenecks.

For most consumers, SATA SSD provides enough performance to do their daily work. And because SATA SSD is cheaper than NVMe SSD, most consumer-grade solid-state storage still uses the SATA interface.

However, enterprises usually process much more data than consumers, for whom the latency caused by SATA connections can be a problem.

Intel 750Series is one of the solid state drives that support NVMe technology.

3. Only SSD runs on NVMe

Because NVMe drives are much faster than SATA, it seems like a good idea to use NVMe with HDD to improve performance. Keep in mind, however, that NVMe stands for the nonvolatile storage interface specification and is specifically designed for nonvolatile storage such as NAND flash memory (although it can also be used for newer non-volatile memory, such as 3D XPoint).

When the system reads data from HDD, it can only read one piece of data at a time. Because it must rotate to locate the correct physical position of the first data block, then rotate again, move to the correct position of the second data block, and so on. On the other hand, flash and other non-volatile storage technologies have no moving parts. This means the system can read data from many different locations at the same time. This is why SSD can take advantage of the parallelism provided by NVMe, while HDD cannot.

4. PCIe and NVMe are related, but they are not the same thing

For many people, the most confusing part of NVMe is its relationship with PCIe. Some vendors use NVMe as a tag to refer to their SSD, others use PCIe tags, and some seem to be able to use these terms interchangeably.

Although PCIe and NVMe are closely related, the two terms refer to slightly different technologies. You can think of PCIe as a physical part of the system. When you plug a NVMe SSD into the server, you need to connect it through a PCIe slot.

By contrast, NVMe is a protocol, a set of hardware and software standards that allow SSD to use the PCIe bus. It can be said that NVMe is the language that allows the storage device to connect to the server, while PCIe is the actual physical connection.

5. NVMe-oF connects SSD to the network

This article focuses on the standard NVMe, which connects SSD directly to the server, but the NVM Express organization has also released the NVMe over Fabric (NVMe- oF) specification, which connects non-volatile storage for block storage to the network. According to the organization, "NVMe- oF defines a common architecture that supports a series of storage network structures for NVMe block storage protocols on top of the network architecture." This includes enabling front-end interfaces in the storage system, extending to a large number of NVMe devices, and extending the accessible distance between NVMe devices and NVMe subsystems within the data center.

The NVMe-oF specification is about 90% the same as the NVMe specification; however, it does use a different transport mapping mechanism. There are also two different variations of NVME-oF: one for RDMA and the other for fibre Channel.

The NVMe-oF specification is also much newer than the NVMe specification, and although some vendors have announced support for this technology, few actually sell NVMe products. It is expected that this technology will become more common in the next few years.

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