What Is SATA Express?

The updated version of SATA increases PC speeds

Serial ATA is a huge success for computer storage. The standardization of the interface allows for easy installation and compatibility between computers and storage devices. The design of serialized communication has reached its limits, however, with many solid state drives capped by the performance of the interface rather than the drive. Accordingly, new standards of communication between a computer and storage drives called SATA Express plug the gap.

SATA or PCI Express Communication

The existing SATA 3.0 specifications are limited to just 6.0 Gbps bandwidth, which translates to roughly 750 MB/s. With overhead for the interface, the effective performance is restricted to just 600 MB/s. Many of the current generations of solid-state drives have reached this limit and need some form of faster interface. The SATA 3.2 specification, of which SATA Express is a part, is a new standard of communication between the computer and devices. It allows devices to pick the existing SATA method, ensuring backward compatibility with older devices, or to use the faster PCI Express bus.

Sata Connector
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The PCI Express bus is most commonly used for communicating between the CPU and peripheral devices, such as graphics cards, networking interfaces, and USB ports. Under the current PCI Express 3.0 standards, a single PCI Express lane handles up to 1 GB/s, making it faster than the current SATA interface. Devices use more than one lane, however. According to the SATA Express specifications, a drive with the new interface can use two PCI Express lanes (often referred to as x2) to achieve a potential bandwidth of 2 GB/s, making it nearly three times the speed of the previous SATA 3.0 hardware.

The New SATA Express Connector

The new interface also required a new connector. It combines two SATA data connectors along with a third slightly smaller connector, which deals with the PCI Express-based communications. The two SATA connectors are fully functional SATA 3.0 ports; a single SATA Express connector on a computer can support two older SATA ports. All of the SATA Express connectors use the full width whether the drive is based on the older SATA communications or the newer PCI-Express. So, one SATA Express handles either two SATA drives or one SATA Express drive.

Because a SATA Express-based drive can use either technology, it must interface with both, so it uses the two ports instead of a third, alternative, one. In addition, many SATA ports link to a PCI Express lane for communicating with the processor. By using the PCI Express interface directly with a SATA Express drive, you are effectively cutting off communication to the two SATA ports linked to that interface anyways.

Command Interface Limitations

SATA communicates data between the device and the CPU. In addition to this layer, a command layer runs on top to send the commands on what should be written to and read from the storage drive. For years, this process was handled by the Advanced Host Controller Interface—it's essentially written into every operating system currently on the market, effectively making the SATA drives plug and play. No extra drivers are needed. While the technology worked well with older, slower technology such as hard drives and USB flash drives, it really holds back faster SSDs. While the AHCI command queue can hold 32 commands, it still can only process a single command at a time because there's only a single queue.

This is where the Non-Volatile Memory Express command set comes in. It features a total of 65,536 command queues, each with the ability to hold 65,536 commands per queue. Effectively, this allows for parallel processing of storage commands to the drive. This isn't beneficial to a hard drive, as it's still effectively limited to a single command because of the drive heads, but for solid-state drives with their multiple memory chips, it can effectively boost their bandwidth by writing several commands to different chips and cells simultaneously.

This is new technology and as a result, it's not built into most of the existing operating systems on the market. In fact, most will need to have additional drivers installed into them so that the drives can use the new NVMe technology. Deployment of the fastest performance for SATA Express drives may therefore take some time. SATA Express supports either of the two methods, so you can still use the new technology now with the AHCI drivers and potentially move to the newer NVMe standards later for improved performance, albeit probably requiring the drive be reformatted.

Other Features in the SATA 3.2 Specs

The new SATA specifications add more than just the new communication methods and connector. Most of them are targeted toward mobile computers but they can also benefit other non-mobile computers as well. The most notable power-saving feature is a new DevSleep mode. It's a new power mode that allows systems in the storage to quasi-hibernate, reducing the power draw when in sleep mode to improve the running times of special laptops—including the Ultrabooks designed around SSDs and low power consumption.

Solid-state hybrid drives also benefit from the new standards, as they have put in a new set of optimizations. In the current SATA implementations, the drive controller determines what items should and should not be cache based upon what it sees bring requested. With the new structure, the operating system tells the drive controller which items it should hold in the cache, which reduces the amount of overhead on the drive controller and improving performance.

Finally, there's a function for uses with RAID drive setups. One of the purposes of RAID is for data redundancy. In the event of a drive failure, the drive is replaced and the data is rebuilt from the checksum. A new process in the SATA 3.2 standards improves the rebuilding process by recognizing which data is damaged versus that which is not.

RAID 10 illustration
By JaviMZN

Implementation and Why It Didn't Catch On Right Away

SATA Express has been an official standard since the end of 2013, but it didn't start making its way into computer systems until the release of the Intel H97/Z97 chipsets in the spring of 2014. Even though motherboards now featured the new interface, no drives at the time of launch used it.

The reason the interface didn't catch on quickly really lies with the M.2 interface. It's used exclusively for solid-state drives that use a smaller form factor. Magnetic-platter drives still have a hard time exceeding the SATA standards. M.2 has a bit more flexibility because it doesn't rely on the larger drives, but it can also use four PCI Express lanes, which means faster drives than the two lanes of SATA Express.

AMD released its Ryzen microprocessors in early March 2017, bringing native support for SATA Express to the AMD Socket AM4 platform.