Nearline and Enterprise SAS vs. NVMe (PCI express) Storage Connections

Nearline and Enterprise SAS vs. NVMe Storage Connections

Most enterprise storage arrays today have backend enterprise SATA (aka Nearline SAS) and enterprise SAS connections running 6gbps or 12gbps where the actual disks and spindles connect to the controllers. The benefits of these enterprise-class communication protocols over standard SATA include:

  • Native command queuing.
  • Dual-redundant multipath I/O connections.
  • Plenty of throughput for each individual spindle.

You would think this is plenty of bandwidth, but now that SSDs are replacing HDDs, there is a case to be made that a newer, better technology can be used. Many individual SSDs can push 500MB/sec on their own. It’s not so much that 12gbps is a bottleneck, but the future of storage isn’t just NAND flash memory. Technologies like PCM and MRAM will easily push the boundaries of being able to move large amounts of data in and out of individual drives, even on the order of 1000x.

How Can We Improve Existing Flash Performance Outputs?

We now might agree that newer technologies are on order for the long term, but even with NAND flash in use today, there could be big improvements in performance by looking at flash differently.

For example, most SSD drives today have multiple NAND chips on the circuit board. If we read and write to these chips in a more parallel fashion, we can get even faster performance. Take existing PCI express-based NAND flash systems out there today, like Fusion-IO or OCZ’s RevoDrive. How can these devices achieve higher throughput and lower latency than a 12gbps SAS connection? For starters, they use the PCI express bus, which removes some controller latency. Taken a step further, NVMe (Non-Volatile Memory Express) is a new specification that can out perform AHCI and even PCIe storage connections.  See the graphic below from communities.intel.com for the latencies of the different stacks comparing the two.

Intel SSD P3700 Series NVMe Efficiency

Image from communities.intel.com.

What Other Benefits Does NVMe Provide?

Some of the other major benefits of NVMe include:

  • Multiple thread usage.
  • Parallel access.
  • Increase in queue depth.
  • Efficiency improvements.

Let’s look at queue depth specifically. AHCI can do 1 queue and 32 commands per queue. NVMe on the other hand can do 64,000 queues with 64,000 commands per queue. Since many SSD drives don’t perform well until there’s a big demand and high queue depth, getting the most performance out of an SSD means hitting it with multiple requests. A 20,000 IOPS drive can often do 80,000-90,000 IOPS with the right queue depth, and newer NAND controller CPUs have more than double the number of channels compared to SATA-based SSD (18 instead of 8), as well as more DDR3 RAM used for cache (1TB instead of 128 or 256GB). So we are starting to see miniature storage array performance in a single SSD “spindle,” which results in capabilities with exceedingly higher performance levels.

One more thing, Intel has a special way to convert a PCIe-based SSD into a standard 2.5” form-factor with the use of the SFF-8639 connector. This connector is what we will start to see in enterprise systems. Wouldn’t it be nice if this connector could use both SATA/SAS or PCIe in the same cable?

How Does NVMe Perform in Independent Tests?

In independent tests, these NVMe-based storage drives are able to hit 200,000-750,000 IOPS using 4KB random reads with queue depths of 128-256. The 4KB random write numbers are lower, from 80,000 – 220,000 at similar queue depths. Sequential read and write performance of many of these drives can easily exceed 2GB/sec, peaking near 3GB/sec for the largest transfer sizes. Average response time peaks at 935 µs, whereas peak latency has a much larger range from 3ms up to 98ms depending on the model, brand and queue depth.

Those super-high latency numbers are proof that IOPS only matter in relation to latency, and it makes sense to choose an SSD drive that offers performance consistency if the application requires it (such as the Micron P320h – 700GB).

What Does NVMe Mean for the Future?

These are strong numbers from a single SSD drive, but the point of all this analysis is two-fold. On the one hand, NVMe is a technology that will lift a potential barrier as NL-SAS and SAS connections eventually become a bottleneck with the release of newer flash-based technologies. On the other hand, much like storage systems of the past decade they are being replaced by newer flash-based systems built from the ground up. We have the opportunity to see a new way of reading and writing to flash that yields even greater performance levels with more parallelism and concurrency, and since we are seeing existing PCI-based SSDs already pushing the limits of SAS, NVMe has a promising future as storage becomes faster and faster.