SATA vs. PCIe: Competition or Coexistence?
14/12/2021 by Roger Griesemer
SATA is dead, long live PCIe! This slogan has been propagated by innovation drivers for the consumer market for years. And not entirely without reason. PCIe with NVMe has several advantages over the SATA interface with AHCI protocol. Nevertheless, SATA also offers some advantages that are particularly useful in special fields of application.
SATA has been mainly developed by Intel for the PC market since 2000. The point-to-point connection and the serial data transfer created room for a constant improvement of the data rate: from the first SATA implementation with 1.5 Gbit/s to SATA Revision 2.x (3.0 Gbit/s) to Revision 3.x with 6.0 Gbit/s, which means approx. 600 Mbyte/s net. SATA was used industry-wide for all storage segments.
In 2003, PCIe revision 1.0 was introduced with a transfer rate of 2.5 Gbit/s. Comparable to SATA, data transfer takes place via a bidirectional connection (lane) with one differential signal pair per direction. Several combined independent lanes increase the available data rate for a connected device. The number of lanes in a slot or on a device is indicated by the abbreviations x1, x4, x8 or x16. As an example, an x8 host slot offers 8 PCIe lanes and can address x1, x4, x8 and x16 devices. The maximum usable number is negotiated between the PCIe host and the device and configured accordingly.
As with SATA, the serial point-to-point connection of the PCIe lanes offered a lot of potential for increasing the transfer speed, as shown below up to and including generation 4.0.
The next generations PCIe 5.0 with 32 and PCIe 6.0 with 64 GT/s are in preparation or development. A 4-lane PCIe 3.1 connection can theoretically perform 32 GT/s while transferring approximately 3.9 GB/s of user data. A 2-lane PCIe Gen3 connection is limited to 1.9 GB/s. However, even a single lane with 980 MB/s is 50% faster than the SATA interface.
AHCI vs. NVMe
In order to avoid customized drivers for each storage medium, open interface standards were created: For SATA devices, the AHCI standard (Advanced Host Controller Interface) evolved over time to support both hard drives and SSDs. Native Command Queuing (NCQ) and SSD-specific extensions such as Trim were added later to AHCI.
NVMe (Non-Volatile Memory Express) was developed for PCIe SSDs - a protocol tailored to the high performance of SSDs with a lean instruction set that can be executed fast. A new feature of NVMe are the so-called namespaces. For this, each host is assigned a corresponding identity (namespace ID), which enables it to address a part of the SSD as if this were an SSD belonging to it alone. This virtualization separates different areas of a physical SSD into non-overlapping regions that can be addressed independently of each other. The namespaces prevent different processes from accessing data from each other. This is particularly important for server virtualization and is especially significant in the automotive industry: Namespaces allow completely different functional areas such as autonomous driving, navigation, and car entertainment to be safely separated from each other and still be mapped on a large SSD.
Effective data rate
It is undisputed that with a correspondingly high number of lanes, the performance of PCIe-SSDs reaches a multiple of that of SATA-SSDs. However, in this case you also need a correspondingly high number of parallel NAND flash channels on the device side: thus, for the theoretically achievable 3.9 GB/s read data rate of a 4-lane PCIe module, 16 parallel 256 Gbit chips would be required with 512 GB capacity. For servers, workstations or high-end applications, this requirement is not a problem at all. However, for industrial applications or boot drives of network systems, capacities of far less than 128 GB memory density are often sufficient, so that SATA is definitely an option. Why it is the better choice at all for some cases will become clear in the following.
This is where one of the undeniable advantages of SATA comes into play: Since SATA only uses one transfer channel, SATA controllers consume less power and thus generate less heat than comparable PCIe modules. Almost all PCIe modules on the market use so-called thermal throttling of the data rate to protect the controller from overheating. With increasing temperatures, the data rate is reduced in several steps, possibly even down to 1/10 of the specified values. In such a case, the advantage of PCIe has become obsolete.
If an SSD is to be used in applications that require an extended temperature range (up to 85 °C), it should be examined in advance whether the temperature management still allows a sufficient data rate. Specifically, fanless systems are sensitive to the internal module temperatures and thus PCIe SSDs do not help them meet the system specification. With SATA modules, on the other hand, you are on the safe side. Mainly because the lower self-heating of the controller does not cause throttling. In addition, SATA allows a data line length of one meter, which means that the SSD can be separated from large heat sources such as the CPU in uncooled systems, while a PCIe module is usually plugged directly into the mainboard. It should be noted that heating leads to unnecessary wear and tear on the flash cells because the stored bits often have to be refreshed at very high temperatures.
In terms of performance and diversity of applications, PCIe offers clear advantages in almost all areas. However, there is still a need for SATA products, especially in industrial and telecommunications applications. The reasons for this are the lower capacity requirements, the extended temperature range and the safe operation even in harsh environmental conditions, such as vibrations. Added to this is the longevity of industrial systems, which results in a continuing demand for SATA SSDs. However, the selection of the right solution should always be made depending on the application and only after detailed consultation.
As a specialist in embedded and industrial storage solutions, Swissbit offers a wide range of SATA products and PCIe solutions in various form factors. We will be happy to help you choose the best solution for your application. Just get in touch.