Unleashing the potential of flash storage with NVMe
Jeroen Dorgelo, Marvell Storage Group [Monday 4 December 2017]
The dirty little secret of flash drives today is that many of them are running on yesterday's interfaces. While SATA and SAS have undergone several iterations since they were first introduced, they are still based on decades-old concepts and were initially designed with rotating disks in mind. These legacy protocols are bottlenecking the potential speeds possible from today's SSDs.
NVMe is the latest storage interface standard designed specifically for SSDs. With its massively parallel architecture, it enables the full performance capabilities of today's SSDs to be realized. Because of price and compatibility, NVMe has taken a while to see uptake, but now it is finally coming into its own.
Serial Attached Legacy
Currently, SATA is the most common storage interface. Whether a hard drive or increasingly common flash storage, chances are it is running through a SATA interface. The latest generation of SATA - SATA III - has a 600 MB/s bandwidth limit. While this is adequate for day-to-day consumer applications, it is not enough for enterprise servers. Even I/O intensive consumer use cases, such as video editing, can run into this limit.
The SATA standard was originally released in 2000 as a serial-based successor to the older PATA standard, a parallel interface. SATA uses the advanced host controller interface (AHCI) which has a single command queue with a depth of 32 commands. This command queuing architecture is well-suited to conventional rotating disk storage, though more limiting when used with flash.
Whereas SATA is the standard storage interface for consumer drives, SAS is much more common in the enterprise world. Released originally in 2004, SAS is also a serial replacement to an older parallel standard SCSI. Designed for enterprise applications, SAS storage is usually more expensive to implement than SATA, but it has significant advantages over SATA for data center use - such as longer cable lengths, multipath IO, and better error reporting. SAS also has a higher bandwidth limit of 1200MB/s.
Just like SATA, SAS, has a single command queue, although the queue depth of SAS goes to 254 commands instead of 32 commands. While the larger command queue and higher bandwidth limit make it better performing than SATA, SAS is still far from being the ideal flash interface.
NVMe - Massive Parallelism
Introduced in 2011, NVMe was designed from the ground up for addressing the needs of flash storage. Developed by a consortium of storage companies, its key objective is specifically to overcome the bottlenecks on flash performance imposed by SATA and SAS.
Whereas SATA is restricted to 600MB/s and SAS to 1200MB/s (as mentioned above), NVMe runs over the PCIe bus and its bandwidth is theoretically limited only by the PCIe bus speed. With current PCIe standards providing 1GB/s or more per lane, and PCIe connections generally offering multiple lanes, bus speed almost never represents a bottleneck for NVMe-based SSDs.
NVMe is designed to deliver massive parallelism, offering 64,000 command queues, each with a queue depth of 64,000 commands. This parallelism fits in well with the random access nature of flash storage, as well as the multi-core, multi-threaded processors in today's computers. NVMe's protocol is streamlined, with an optimized command set that does more in fewer operations compared to AHCI. IO operations often need fewer commands than with SATA or SAS, allowing latency to be reduced. For enterprise customers, NVMe also supports many enterprise storage features, such as multi-path IO and robust error reporting and management.
Pure speed and low latency, plus the ability to deal with high IOPs have made NVMe SSDs a hit in enterprise data centers. Companies that particularly value low latency and high IOPs, such as high-frequency trading firms and database and web application hosting companies, have been some of the first and most avid endorsers of NVMe SSDs.
Barriers to Adoption
While NVMe is high performance, historically speaking it has also been considered relatively high cost. This cost has negatively affected its popularity in the consumer-class storage sector. Relatively few operating systems supported NVMe when it first came out, and its high price made it less attractive for ordinary consumers, many of whom could not fully take advantage of its faster speeds anyway.
However, all this is changing. NVMe prices are coming down and, in some cases, achieving price parity with SATA drives. This is due not only to market forces but also to new innovations, such as DRAM-less NVMe SSDs.
As DRAM is a significant bill of materials (BoM) cost for SSDs, DRAM-less SSDs are able to achieve lower, more attractive price points. Since NVMe 1.2, host memory buffer (HMB) support has allowed DRAM-less SSDs to borrow host system memory as the SSD's DRAM buffer for better performance. DRAM-less SSDs that take advantage of HMB support can achieve performance similar to that of DRAM-based SSDs, while simultaneously saving cost, space and energy.
NVMe SSDs are also more power-efficient than ever. While the NVMe protocol itself is already efficient, the PCIe link it runs over can consume significant levels of idle power. Newer NVMe SSDs support highly efficient, autonomous sleep state transitions, which allow them to achieve energy consumption on par or lower than SATA SSDs.
All this means that NVMe is more viable than ever for a variety of use cases, from large data centers that can save on capital expenditures due to lower cost SSDs and operating expenditures as a result of lower power consumption, as well as power-sensitive mobile/portable applications such as laptops, tablets and smartphones, which can now consider using NVMe.
Addressing the Need for Speed
While the need for speed is well recognized in enterprise applications, is the speed offered by NVMe actually needed in the consumer world? For anyone who has ever installed more memory, bought a larger hard drive (or SSD), or ordered a faster Internet connection, the answer is obvious.
Today's consumer use cases generally do not yet test the limits of SATA drives, and part of the reason is most likely because SATA is still the most common interface for consumer storage. Today's video recording and editing, gaming and file server applications are already pushing the limits of consumer SSDs, and tomorrow's use cases are only destined to push them further. With NVMe now achieving price points that are comparable with SATA, there is no reason not to build future-proof storage today.
(Jeroen Dorgelo is director of strategy at Marvell Storage Group)
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