Tech Talk
SAS, SATA or PCIe: Know your Interface
Last updated 5 September 2022
With so many varieties of storage technology available, it can be quite bewildering in choosing the right one. The particular computer or storage device you have will also dictate which interface you can use. There’s a little bit of wiggle room here, but in general, you must match the storage interface to the interface used in the backplane of your server or storage controller.
Choosing your interface ultimately comes down to what you are using your storage for. The most common type of interface is SATA which can be found on most SSDs and embedded technology. SATA has been around for years with the latest incarnation SATA 3, launched in 2009. Another interface, SAS, is aimed at the datacentre market and HDD storage in particular but there is a new kid on the block, PCIe which looks to shake things up.
Although we say ‘new’, PCIe has been around since 2004 but it hasn’t been until recently that it has come to the fore, mainly due to 3D NAND flash making PCIe-based storage more appealing and cheaper. However, we must not forget the SAS interface and although old, still has a valuable role to play.
SAS
Serial Attached SCSI or SAS is a legacy interface that is typically used for transferring data to external storage commonly found in large datacentres.
SATA
SATA is the market incumbent and dominant interface for connecting an SSD to the PC. It employs the command protocol AHCI (it also supports IDE) which was built with slower spinning disks in mind rather than flash memory. SATA transfer rates begin at 150 MB/s and max out at 600 MB/s for third-generation technology. For most consumers, this is more than fast enough.
PCIe
PCI Express supersedes SATA as the latest high bandwidth interface. Entry-level PCIe SSD speeds are two to three times faster than the older generation of SATA 3.0 SSDs mainly due to the number of channels contained by each to transfer data (roughly 10 for SATA and 25 for PCIe). However, depending on usage, real-world benchmarks may not reflect this massive gain due to bottlenecks elsewhere in the PC.
NVMe is the latest high-performance and optimised protocol which supersedes AHCI and compliments PCIe technology. It offers an optimised command and completion path for use with NVMe-based storage. It was developed by a consortium of manufacturers specifically for SSDs to overcome the speed bottleneck imposed by the older SATA connection.
Side by side what are the pros and cons of SAS v SATA v PCIe?
The table below gives a basic overview of the key differences.
| SAS | SATA | PCIe | |
|---|---|---|---|
| Bandwidth | Low | Medium | High |
| Queue Command Depth | 254 | 32 | 65,536 |
| Advantages | Fast data transfer speeds, full duplex data transfer, 8m cable length, dual-ported, support multiple initiators | Half duplex data transfer, inexpensive, large capacities, low power consumption, single ported, SCSI commands | Super fast speeds -8x faster than SATA, latency reduction |
| Disadvantages | Low capacities, old technology, very expensive | low transfer speeds, 1m cable length | Limited capacities, expensive, ATA commands |
| Cost | ££££ | £ | £££ |
| Ideal for | Datacentres, mission-critical enterprise applications | Personal & desktop use, tablets & datacentres, a cheap upgrade from HDD | Some datacentre environments, Video editing, financial modelling, and simulation. High-end gaming |
How do SATA and SAS work?
Let’s start with a few basics about SATA and SAS. Serial Advanced Technology Attachment (more commonly referred to as SATA) is a type of hardware interface that allows the transfer of data to and from HDDs and SSDs.
While fine for everyday computing, in a RAID server array or datacentre environment, often a better alternative has been to use ‘SAS’ drives, which stands for Serial Attached SCSI. This is another type of interface that, again, is usable either with HDDs or SSDs. ‘SCSI’ stands for Small Computer System Interface (which is why SAS drives are sometimes referred to as ‘scuzzy’ drives). SAS has increased IOPS (Inputs Outputs Per Second) over SATA, meaning it can read and write data faster. This has made SAS an optimal choice for systems that require high performance and availability. In computing, SAS is a point-to-point serial protocol that moves data to and from computer-storage devices such as hard drives and tape drives... SAS, like its predecessor, uses the standard SCSI command set.
On an enterprise level, SAS prevails over SATA, as SAS has more over-provisioning to prolong write life and has been specifically designed to run in environments that require constant drive usage, such as datacentres.
Protocol Interface
With each interface comes a protocol or controller interface. For example, Advance Host Controller Interface, or AHCI is a hardware mechanism that allows the software to communicate with SATA-based devices.
PCIe
Like SATA there have been many developments in the PCIe (Peripheral Component Interconnect Express) interface. Often seen abbreviated as PCIe or PCI-E, is a standard type of connection for internal devices in a computer. PCIe has been around for several years but is seeing increasing adoption owing to its speed.
Because of SATA’s 0.6GB/s ceiling (see above table), PCIe is starting to supersede SATA as the latest high-bandwidth interface. A PCIe connection consists of one or more data-transmission lanes connected serially. Each lane consists of two pairs of wires, one for receiving and one for transmitting. You can have one, four, eight or sixteen lanes in a single PCIe slot, denoted as x1, x4, x8, or x16.
PCIe technology enables interface speeds of up to 1GB/s per client lane (PCIe 3.0), versus SATA, 0.6GB/s. More lanes from SATA require more SATA devices, but PCIe bandwidth can be scaled up to 16 lanes on a single device. As technology evolves we are now seeing the latest incarnations of PCIe, notably 5.0 which has incredibly fast transfer rates of 32GT/s. PCIe 6.0 & 7.0 are already in different stages of development with 7.0 slated for release in 2025
While computers may contain a mix of various types of expansion slots, PCIe is considered the standard internal interface. Many computer motherboards today are manufactured only with PCIe slots, so progression to PCIe is inevitable. The next generation of home and business computing is starting to switch to PCIe storage, be it older versions, mainly 3.0 giving much faster speeds than SATA-based storage
PCIe improvements continue with 3.1 available in the marketplace. 4.0 was officially launched in late 2017 (but first announced in 2011) and doubles the data transfer rate available in PCIe 3.0, (which debuted in 2010). 5.0 was released in 2019 and 6.0 was released in 2022.
What does the future hold?
SATA SSDs will still technically have their place as higher capacity flash storage for the foreseeable future. However, if speed isn’t a priority, any HDD will continue to be a cost-effective alternative with its low cost per gigabyte. Many new technologies are hitting the market for SSD storage, including M.2 PCIe SSDs, which offer vastly increased performance and are widely cited to be the new standard for the consumer laptop market in 2017 and beyond. In addition, NVMe is slowly replacing SAS SSDs for enterprise-grade/performance-based IT as datacentres look to enhance user experience.
NVMe
NVMe (Non-Volatile Memory Express) is an open logical device interface specification for accessing non-volatile storage media attached via a PCIe bus. NVMe is a fairly new protocol for accessing high-speed storage media that brings many advantages compared to legacy protocols. But why is it important for data-driven businesses?
As businesses contend with the perpetual growth of data, they need to rethink how data is captured, preserved, accessed, and transformed. Performance, economics, and endurance of data at scale are paramount. NVMe is having a great impact on businesses and what they can do with data, perfect for data centres and data-hungry applications.