NVMe vs SATA: What is the difference?

SATA SSDs

Serial ATA (SATA) technology was introduced back in 2000 as an improvement to the existing Parallel ATA technology, which was hampered by cable size, cost, performance and functionality. Both technologies were sufficient for hard disk drives (HDDs), which were substantially less capable than today’s solid-state drives (SSDs). The advent of SATA-based SSDs demonstrated that the ATA bus had reached its performance limit. Where HDDs were only able to achieve 50-120 MB/s in write performance, SSDs could saturate the SATA bus at 550 MB/s. In spite of the bus limitation, it’s common to see an overall system performance improvement of 10-15 times when using SATA-based SSDs in place of legacy HDD technology.

NVMe SSDs

Non-Volatile Memory Express (NVMe) technology was introduced in 2011 to address the various bottlenecks of the SATA interface and communication protocols. NVMe technology utilises the PCIe bus, instead of the SATA bus, to unlock enormous bandwidth potential for storage devices. PCIe 4.0 (the current version) offers up to 32 lanes and can, in theory, transfer data at up to 64,000MB/s compared to the 600MB/s specification limit of SATA III. The NVMe specification also allows for 65535 command queues, which can have up to 65536 commands per queue. Recall that SATA-based SSDs are limited to a single queue with a depth of only 32 commands per queue. NVMe technology creates massive potential for storage devices through increased efficiency, performance and interoperability on a broad range of systems. It is commonly believed that the technology will become the new industry standard.

SSD performance at a glance

NVMe is a storage protocol designed specifically with SSDs in mind. With the elimination of the intermediating SATA HBA layer, NVMe allows SSDs to communicate directly with the CPU via the PCIe bus, opening channels for ground-breaking performance improvements. To put it into perspective, the performance limit of the SATA III bus is 6Gb/s, meaning a SATA SSD can offer a max of 550MB/s of throughput after overhead. A single PCIe 3.0 lane can offer 1GB/s (bidirectional) of throughput, so a PCIe 3x4 SSD can reach a throughput of up to 4GB/s read/write. That goes up to 8GB/s (bidirectional) for PCIe Gen 4X4 SSDs. The performance limitation here moves from the protocol to the NAND media, which has been undergoing tremendous development in recent years, allowing manufacturers to squeeze the highest density and performance into the smallest form factors.

Protocol latency is also greatly reduced with NVMe due to the shortened and optimised datapath, allowing lower latency than SATA/SAS. The management of queues in NVMe devices is also handled more efficiently by the CPU due to the I/O processing doorbell signalling which significantly reduces CPU overhead. In addition, NVMe devices are supported by most major operating systems due to the massive development effort carried out in the past decade.

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