Summary

This document is a straightforward guide to help PowerEdge customers choose the most appropriate SSD type, based on their business needs and goals. 

As new generations of CPUs and servers are released, they frequently bring new technologies such as increased PCIe bus speeds and new storage formats, such as the EDSFF E3.S form factor for NVMe PCIe 5 Solid State Drives (SSDs), as released in early 2023. PowerEdge customers can optimize their local storage configurations based on their applications and business needs. Multiple factors must be taken into consideration to make an informed decision, such as workload demands, budget, scale, and even roadmap. Still, when all of these factors are understood, it can be difficult to determine the best choice of SSD interface among NVMe, SAS, Value SAS, and SATA.

This DfD (Direct from Development) tech note is provided to simplify and guide customers in their choice of SSD. We hope customers will find it to be a valuable resource when it becomes unclear which storage medium is the optimal choice. First, let’s summarize the history and architecture of the NVMe, SAS, Value SAS, and SATA SSD interfaces:

NVMe (Non-Volatile Memory Express)

Since it came to market in 2011, the NVMe interface remains the class of flash storage with the highest performance. The driving architectural differentiator of NVMe is that it uses the PCIe interface bus to connect directly to the CPU and streamline the data travel path. This design contrasts with SAS and SATA, which require data to first traverse to an HBA disk controller before reaching the CPU. By removing a layer from the stack, the travel path is optimized and produces reduced latency and improved performance. Scalability is also significantly improved, because NVMe drives can go beyond the traditional four lanes by using lanes from the same “pool” of lanes connected to the CPU. EDSFF including EDSFF E3.S are the next generation of NVMe SSDs. These form factors enable higher server storage density. Furthermore, NVMe performance continually improves as each new generation of the PCIe standard becomes available.

Figure 1.  Latest Dell PowerEdge R7625 with 32 x E3.S drives

SAS (Serial Attached SCSI)

The SAS interface was released a few years after SATA and introduced new features that are beneficial for modern workloads. Instead of building upon the ATA (Advanced Technology Attachment) standard used in SATA, SAS serialized the existing parallel SCSI (Small Computer System Interface) standard. SAS cable architecture has four wires within two cables, creating more channels available for moving data and more connectors available for use by other devices. Furthermore, the channels are full duplex, allowing for reads and writes to traverse concurrently. Improved reliability, error reporting, and longer cable lengths were also introduced with SAS. Value SAS is often alongside SAS using the same interface but using lower performance devices, giving customers the technical benefit of SAS at a lower a price point. SAS improvements are made to this day, with SAS4 (24G) now available in certain supported PERC 12 (PowerEdge Raid Controller) configurations. For this reason, SAS still remains valuable and relevant within the market.

SATA (Serial Advanced Technology Attachment)

The SATA interface was released in 2000 and is still commonly adopted within modern servers because it is the most affordable of the SSD interface options. It replaced parallel ATA with serial ATA, which resolved various performance and physical limitations at that time. The SATA cable architecture has four wires within one cable—two for sending data and two for receiving data. These four channels are half-duplex, so data can only move in one direction at a time. At 6Gb/s, SATA write speeds are sufficient for storing information, but its read speeds are slow compared to more modern interfaces, which limits its application use for modern workloads. The last major SATA revision was in 2008, and SATA will not see further advancement in the future.

Figure 2.  Random 4KiB 70% read / 30% write IOPS variances for each storage interface

Table 1 lists key metrics for five storage-drive types most commonly attached to PowerEdge servers: Enterprise NVMe, Data Center (DC) NVMe, Enterprise SAS, Value SAS, and SATA. This comparison helps clarify which storage interface type is most applicable to specific business needs and goals.

Table 1.  Ranking performance metrics of Enterprise NVMe, DC NVMe, Enterprise SAS, Value SAS, and SATA drives

Performance: Performance can be measured in various ways. For this example, Random 4 KiB 70/30 (70% reads, 30% writes) data was compared and published here by Dell, with higher IOPS being better. Figure 2 illustrates the following IOPS performance variances:

• E3.s NVMe Enterprise class drives produce 1.48x more IOPS than Enterprise NVMe SSDs. 
• Enterprise NVMe SSDs produce 1.13x more IOPS than DC NVMe SSDs. 
• DC NVMe SSDs produce 1.99x more IOPS than Enterprise SAS SSDs. 
• Enterprise SAS SSDs produce 1.42x more IOPS than Value SAS SSDs. 

Lastly, Value SAS SSDs produce 2.39x more IOPS than SATA. Random 4KiB 70% read / 30% write IOPS variances for each storage interface

Latency: The NVMe protocol reduces the number of touchpoints that data must travel (bypassing the HBA) to reach the CPU. It also has less overhead, giving it significantly lower latency than SAS and SATA. The SAS protocol is full-duplex (as opposed to half-duplex) and offers two channels (as opposed to one) for data to use, giving it over 50% lower latency than SATA.

Price: According to Dell pricing in Q1 2022, SATA SSDs are the least expensive storage interface, at ~0.9x the price of Value SAS SSDs. Value SAS SSDs are ~0.85x the price of DC NVMe SSDs. DC NVMe SSDs are ~0.85x the price of Enterprise SAS SSDs. Enterprise SAS SSDs are ~0.97x the price of Enterprise NVMe SSDs. Pricing is volatile and these number variances are subject to change at any time.

Performance per price: PowerEdge customers who have not identified which metric is most important for their business goals should strongly consider performance (IOPS) per price (dollar) to be at the top of the list. Because NVMe has such a significant performance lead over SAS and SATA, it is easily the golden standard for performance per price. DC NVMe SSDs have the best performance per price, followed closely by Enterprise DC NVMe SSDs, followed by Value SAS SSDs, followed closely by SAS SSDs, followed by SATA SSDs. This tech note gives more performance/price detail.

Scalability: Currently, NVMe shows the greatest promise for wider-scale implementation due to the abundance of lanes that can be available with low overhead. However, it can be a costly investment if existing data center infrastructures must be upgraded to support the NVMe I/O protocol. SAS is more flexible, because SAS expanders are cost-effective, and most data center infrastructures already have the required hardware to support it. However, SAS does not have the potential to scale out as aggressively as NVMe. SATA does not scale well with SSDs.

Ongoing development: The NVMe interface has consistent and substantial advancements year-over-year, including updates such as NVMe 2.0b (released in Oct. 2022) and PCIe 5.0 (released on Intel CPUs in Jan. 2023). The SAS interface also has regularly cadenced updates, but the impact is mostly marginal, except for the recent SAS4 (24G) update. There are no plans to extend the capabilities of the SATA interface beyond the current limitations.

Assigning these ranks for each storage interface and metric, and explaining why the rank was given, will make it easier to understand which drive type will be the most valuable in relation to business needs and goals.

Guidance in accordance with business goals

Each business is unique and will have different requirements for their storage drives. Factors such as intended workload, business size, plan to scale, budget, and so on, should be considered to make a confident investment decision. Although this decision is ultimately up to each business, we provide the following guidelines to help businesses that are still undecided to make an educated choice:

Enterprise NVMe SSD: Businesses that desire maximum performance and have a flexible budget should consider purchasing Enterprise NVMe SSDs. Storage I/O heavy workloads such as HPC or AI will immediately benefit from the additional cache gained from the non-volatile nature of this storage interface. The fast-paced performance growth seen in Enterprise NVMe SSDs will also allow smaller workloads like databases or collaboration to easily keep up with the ever-increasing size of data. Ultimately, because Enterprise NVMe undergoes consistent valuable changes every year, such as performance increases and cost reduction/optimization, we recommend futureproofing your data center with it.

DC NVMe SSD: Businesses that desire a budget conscious NVMe solution, in addition to the greatest value, should consider purchasing DC NVMe SSDs. These drives have the same value proposition as for Enterprise NVMe SSDs, but with a sizeable price reduction (0.83x) and performance hit (0.86x). Businesses that want to get the best value will be pleased to know that DC NVMe drives have the best performance-per-price.

Enterprise SAS: Businesses that desire to continue using their existing SCSI-based data center environment and have maximum SAS performance should consider purchasing Enterprise SAS SSDs. Although the Enterprise SAS interface does not currently have any ranking leadership for performance or pricing, it is established in the industry as highly reliable, cost-effective to scale, and shows promise for the future, with 24G available. Enterprise SAS SSDs will adequately handle medium-duty workloads, such as databases or virtualization, but will operate best when mixed with NVMe SSDs if any heavy-duty workloads are included.

Value SAS: Businesses that desire a budget-conscious SAS solution should consider purchasing Value SAS SSDs. These drives have the same value proposition as for Enterprise SAS SSDs, but with both a sizeable price reduction (0.73x) and performance hit (0.71x). For this reason, it has a slightly lower performance-per-price than Enterprise SAS, and is therefore more of a “value” play when compared to SATA. This storage interface has a purpose for existing though, because small-to-medium businesses with a smaller budget can leverage this lower-cost solution while still receiving the many benefits of the SAS interface.

SATA: Businesses that desire the lowest price storage interface should consider purchasing SATA SSDs. However, caution should be applied with this statement, because there is currently no other value proposition for SATA SSDs, and the price gap for these flash storage interfaces has been shrinking over time, which may eventually remove any valid reason for the existence of SATA. With that said, SATA is currently still a solid choice for light workloads that are not read-heavy.

Figure 3.  Latest Dell PowerEdge MX760c with 8 x E3.S drives per sedge

Conclusion

The story of competing NVMe, SAS, and SATA storage interfaces is still being written. Five or more years ago, analysts made the argument that although NVMe has superior performance, its high cost warranted SAS the title of ‘best value for years to come’. What we see today is a rapidly shrinking price gap for all of these interfaces. We observe that SATA performance has fallen far behind SAS, and very far behind NVMe, with no plan to improve its current state. We also see NVMe optimizing its performance and price-point to yield more market share every year. Most importantly, we expect rapid growth in the industry adoption of heavier workloads and ever-increasing data requirements. Both storage drive and industry trends lead us to believe that the best option for any business desiring to build a future-proofed data center would be to begin making the investment in NVMe storage. However, the remaining types of storage still hold value for varying use cases. It is the customer’s choice about which storage type is best for their business goals. We hope this guide has helped to clarify the available options.