A Deep Dive into SSDs: SATA, SAS and PCIe

If you’re among the many enterprises planning to utilize the power of flash-based solid-state drives (SSDs), one of the most predominant questions you might have is: which SSD is best for incorporating flash?

If one were to research this topic hoping to find a concise answer, your hopes would be promptly dashed as you found yourself slipping further into the rabbit’s hole that is enterprise data storage. While there are more than several SSD interfaces, most people new to exploring the market may have heard a thing or two about Serial Attached SCSI (SAS), Serial ATA (SATA), or Peripheral Component Interconnect Express (PCIe). However, out of respect for those readers who have very little to zero knowledge about flash-based SSDs, we’ll recap some information already covered in previous articles published by RAID Inc.

Falling NAND Prices Bring PCIe and SATA SSDs to a Near Price Parity

Flash-based SSDs are steadily growing in popularity beyond the consumer level. Sanjay Mehrotra, CEO, President, and Director of Micron Technology, Inc. reported that Client NVMe SSDs represented nearly 75 percent of client SSD bits shipped to data center customers in this first fiscal quarter of 2020. Mehrotra also pointed out that this is a drastic increase compared to shipping virtually no client SSD bits the first fiscal quarter of 2019.

As with anything involving technology, many discernible and indiscernible factors are influencing the solid-state drive market with the most obvious being a sharp price reduction in NAND flash which is used in flash memory cards and SSDs. This drastic shift in price has allowed manufacturers to start tailoring innovative solutions for data center and enterprise use cases. On the flip side, the lower cost of client SSDs encourage original equipment manufacturers (OEMs) to incorporate them in consumer PCs and a growing number of enterprise data storage devices.

Understanding the Difference Between SATA and NVMe

Non-Volatile Memory Express (NVMe) drives are specially designed to be used in conjunction with SSDs. The NVMe interface protocol is a significant improvement over older hard disk drives (HDDs), but it’s also proving to be a whole lot faster than the SATA computer bus interface. For example, PCIe 3.0 offers a maximum transfer speed of around 985 Mbps per lane, with NVMe drives capable of utilizing four lanes and achieving a maximum speed of 3,940 Mbps (3.9 Gbps).

All of that speed potential may sound good but it doesn’t come without some downsides that must be considered. Firstly, even with the drop in prices, it totally depends on high-performance SSDs and still makes for an expensive option over SATA. Then there is the fact that NVMe running on client PCs requires devices to be in the M.2 format. This will greatly limit your drive selection. For those operating legacy devices, don’t expect too much support for older machines. Lastly, NVMe devices are the most cost-effective solution for those looking to store large amounts of data, especially when one considers NVMe drives are known to lose data from time to time.

The Development and Evolution of Solid State Drives (SSDs)

In 1878, the StorageTek STC 4305 was introduced as being the first semiconductor storage device that was compatible with a hard drive interface. The STC 4305 was reportedly seven times faster and half the price of the IBM 2305 fixed-head disk drive. It later switched to the dynamic random-access memory (DRAM). In the late 1980s, Zitel introduced its family of DRAM-based SSD products to be used with UNIVAC and Perkin-Elmer systems. In 1989, Sanjay Mehrotra and Eli Harari, the founders of SanDisk, and a third person named Robert D. Norman filed a patent for a flash-based SSD after realizing the great potential of using flash memory as an alternative to a hard drive.

In 1991, SanDisk shipped the first commercially manufactured flash-based SSD (a 20 MB SSD) in the Personal Computer Memory Card International Association (PCMCIA) configuration. In those days, these flash-based SSD PC Cards sold OEM for approximately $1,000. IBM used them in its innovative ThinkPad laptop which was released onto the market in 1992. Since then, flash-based SSDs have changed a lot; we’ve gone from a 20 MB flash-based SSD to 8 TB 16-lane PCIe 4.0 SSD with 15,000 Mbps sequential read and 15,200MB/s sequential write speeds, as was demonstrated by Taiwanese manufacturer and distributor Gigabyte Technology.

Even though flash-based SSD has been on the market for nearly 30 years, Big Data analytics and artificial intelligence (AI) has thrust flash technology into the limelight. More organizations are beginning to adopt flash-based SSD to take advantage of its performance, but many have yet to do so and wonder which SSD is best for incorporating flash: PCIe, SAS, or SATA.

Peripheral Component Interconnect Express (PCIe)

The first SSD drive on our list is the PCIe. It’s among one of the most desirable SSDs on this list—not to mention the most expensive. The reason for this boils down to performance. Think of a PCIe SSD as a more uninterrupted link to the motherboard by bypassing the SAS or SATA controllers and plugging directly into the backplane. This means that a PCIe SSD is basically the backplane, resulting in exceptional speeds.

PCIe is the standard motherboard interface for technologies such as graphics cards, hard disk drives (HDDs), solid-state drives (SSDs), Wi-Fi, and Ethernet hardware. PCIe is defined as a high-speed serial computer expansion bus standard and offers numerous improvements over the older bus standards like PCI, PCI-X, and AGP.

One of the major improvement includes a smaller physical footprint, higher maximum bus throughput, better performance scaling for bus devices, a lower I/O pin count, native hot-swap functionality, a more reliable reporting mechanism (Advanced Error Reporting, AER) and error detection, and hardware support for I/O virtualization. PCIe 3.0 supports 1x, 4x, 8x,16x, and 32x lanes at an effective transfer speed of 985 MB/s per lane. That is a possible transfer speed of up to 15.76 GB/s. In May 2019, the official PCIe 5.0 standard was published, promising 128 GBps of throughput.

SATA and SAS SSDs connect to PCIe backplanes as well, but they couple physical hardware form-factors with software protocols, slowing down data transfers. PCIe focuses on electrical signaling, producing SSD flash drives that were designed with PCIe strictly in mind. Furthermore, due to its parallelism and exceptional speeds, PCIe-based SSDs offer a throughput more suited to the NAND format. And thanks to Moore’s Law, prices for PCIe-based SSDs and SATA-based SSDs with the same capacity have since come into parity.

Serial AT Attachment (SATA)

SATA is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives (HDDs), optical disc drives (ODDs), and solid-state drives (SSDs). SATA superseded the Parallel ATA (PATA) standard and became the dominant interface for storage devices. The term “SATA” is derived from the Serial ATA International Organization (an independent, non-profit organization) which is responsible for providing guidance and support for the computer industry in implementing SATA specifications. These standards are then published by the INCITS Technical Committee T13, AT Attachment (INCITS T13).

For the above-stated reason, most SSDs are manufactured based on the SATA interface format. SATA III SSD (introduced in 2009) is the most current version and is capable of reading data at 6 Gbps (600 Mbps). SATA III SSD writers are only slightly slower than that but still a whole lot faster than any HDD on the market (which is approximately 120 Mbps). Due to their longlasting popularity and the number of companies manufacturing SATA solid-state drives, they’ve remained one of the most affordable options on the market.

However, as NAND flash memory prices continue to fall and the price gap between PCIe–based and SATA-based SSDs continue to shrink, we will begin to see a bigger shift towards PCIe SSDs in 2020.

Serial Attached SCSI (SAS)

The SAS and SATA interfaces are quite similar in that they both do roughly the same job. However, since each one is built with different hardware, they both are suited for different workloads. For example, SATA is less expensive but is better suited for desktop file storage. On the other hand, SAS-based SSDs are pricier but better suited for use in process-heavy computer workstations or servers. SAS III SSDs offer upwards of 12 Gbps, twice faster than SATA-based SSD flash drives. Furthermore, SAS-based SSDs feature one to three drive writes per day (DWPD) with capacities from 800 GB to 6.4 TB and capacities between 960 GB and 15.36 TB.

SAS SSDs are attractive to enterprises because they support multiple data paths, superior data-path redundancy, and high availability. In addition to that, SAS SSDs sit naturally in dual-controller based storage arrays, making them a great choice for most software-defined storage solutions on the market today.

Another thing about SAS drives versus SATA drives is that they deliver less hardware overhead (the term “overhead” refers to any combination of indirect or excessive computation time, memory, bandwidth, or other resources that are required to perform a specific task). This becomes critical when the number of input/output operations per second (IOPS) and the latency of data exchange is an issue. Compared to SATA-based SSDs, SAS-based SSDs deliver better overall end-to-end data integrity and feature a more highly configurable reporting structure.

Lastly, if an array or server supports an SAS interface, SAS or SATA-based SSDs can be plugged into it and both will work. However, this isn’t so with arrays or servers that feature a SATA backplane; only SATA SSDs will work.