Different Types of Server SSD Interface

Generally, server hard drives mainly include three types: hard disk drives (HDDs), solid-state drives (SSDs) and hybrid disk drives (HDDs). When it comes to computer storage, HDDs are probably mentioned most of the time. However,SSDs enable faster information processing and better computer performance with lower power. The following will focus on three server SSD interfaces and their differences.

Types of Server SSD Interfaces

As an improved storage device, server SSD uses persistent flash memory to retain information, which is less prone to physical damage from wear and tear. Generally, there are three types of server SSD interfaces: Serial Advanced Technology Attachment (SATA), Serial Attached SCSI (SAS), and Non-Volatile Memory Express (NVMe).

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SATA (Serial Advanced Technology Attachment)

SATA, short for Serial Advanced Technology Attachment, is the oldest and most cost-effective for SSD interfaces compared to the other two server interface types. As a computer bus, the main function of SATA interface is to transmit data between motherboard and storage devices like hard disks over a high-speed serial cable, thus supporting hot swapping— the ability to replace system components without shutting down the system.

Different from its predecessor--PATA, SATA is faster and can write to disk at 6 Gb/s interface rate and 600 MB/s throughput. It supports backwards compatibility both with hardware and software.

However, SATA is a half-duplex interface, which means it can only use one channel/direction to transfer data and cannot perform read and write functions at the same time, which may lead to network bottlenecks and performance delays. Besides, SATA also has less functionality for error recovery and reporting than SAS and NVMe interfaces.

SAS (Serial Attached SCSI)

SAS or Serial Attached SCSI is a new generation of SCSI technology. Like SATA interface, SAS also adopts serial technology for higher transmission speed and supports hot swapping. It is used to support 128 direct point-to-point data transmission between hardware devices for enterprise-level storage solutions. Besides, SAS is a full-duplex interface and supports simultaneous read and write functions, which can transfer data bi-directionally at speeds up to 12Gb/s.

SAS interface is generally compatible with the SATA interface. The backplanes of SAS systems can be connected to both dual-port, high-performance SAS drives and high-capacity, low-cost SATA drives. Usually, SAS provides more throughput and supports larger capacities and higher performance capabilities than SATA interface.

But SAS is not a perfect solution either. It still has several shortcomings, such as high price and poor value for money, etc.

NVMe (Non-Volatile Memory Express)

Non-Volatile Memory Express (NVMe) interface is a type of SSD interfaces that connects to a PCI Express (PCIe) slot on the motherboard. It is an improvement over the SATA and SAS interfaces. Similar to SAS interface, NVMe also has a full-duplex capability, but far exceeds SAS in terms of data transfer power, driving an interface rate of 32 Gb/s and a throughput of 3.9 GB/s.

Located directly between device drivers and PCIe, NVMe is able to achieve high scalability, security, and low latency data transmission, which can deliver up to 4 times the I/O operations per second than SAS interface. Therefore, NVMe SSD interfaces are increasingly replacing SATA to become a better choice for high-performance interface in enterprise-level applications.

However, the drawbacks of NVMes are also very obvious. First of all, it is the most expensive SSD interface among other types. It is also not cost-effective when large amounts of memory need to be stored. What's more,compatibility is also an issue that plagues NVMe interface. Data servers with NVMe interface should use M.2 format, which is quite limited compared to other storage solutions.

Comparison of Different Types of Server Interfaces

Interface type

Typically, there are two standard interface protocols of network servers: NVMe and Advanced Host Controller Interface (AHCI). The server NVMe interface uses the NVMe interface protocol, while the server SATA and SAS interfaces use the AHCI interface protocol and also support the Integrated Development Environment (IDE).

Besides, server interface pots also vary. The NVMe interface only uses PCIe ports, SAS uses M.2, U.2, mSATA, SATA, SAS, PCI-E, SATA express, and IDE ports, and SATA uses SATA, U.2, and M.2 ports. What's more, the PCIe bus channels used by the NVMe SSD interface can be directly connected to the CPU, while the other two cannot.

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Read/Write speed

As the figure shows above, SATA III used on modern motherboards has a maximum throughput of 600MB/s and interface rate of 6 Gb/s, while SAS interface supports the transfer rate of up to 12Gb/s since the latter is a faster all-around technology than the former.

As for NVMe interfaces, they have multiple information buses and can provide read and write speeds of 2000MB/s. Besides, since it can be directly connected to the CPU without the limitation of APA interface, input/output operations (IOP) per second can be up to 4x faster.

Scalability＆Performance

NVMe interfaces have the best scalability and performance among these three interfaces. Since NVMe utilizes PCIe slots, it can transfer 25 times the amount of data compared to equivalent SATA products. What's more, it surpasses and utilizes the traditional four lanes found in most PCIe SSD interfaces to boost performance.

The scalability of the SATA interface is significantly lower than that of other storage interfaces. Due to its limited port capabilities, the data transfer rate is limited, which affects the drive's performance. The SAS interfaces provide greater scalability, theoretically supporting up to 16,384 devices.

Latency

As a half-duplex interface, SATA can only use one channel/direction to transfer data and will easily experience network bottlenecks and performance delays. The SAS interface performs better than SATA in terms of latency because of its full-duplex interface.

However, the NVMe interface has the lowest latency. Relying on a native PCIe controller to realize direct connection to the CPU, which means there is no requirement for CPU to read registers while executing commands, thus reducing latency to the largest degree.

Price

As is well-known, the faster and better performance the drive, the higher the price. From the above description, it is clear to see that SAS and NVMe are better than SATA in terms of read/write speed, scalability, performance and latency.

Besides, because of the complex hardware, SAS and NVMe are more expensive than SATA interface, but they are also more durable than SATA to some extent.

Conclusion

To sum up, SATA, SAS and NVMe are three main SSD interfaces which vary in several respects. In comparison, SATA interfaces are easy-management and can provide the most cost-effective data storage solutions for network servers, while SAS and NVMe interfaces are faster and have lower latency, better scalability and performance.