As databases grow and query times increase, the need for faster enterprise storage becomes critically important. Enterprise solid-state device (SSD) technology continues to mature and evolve at a very rapid pace, especially within mission-critical enterprise data centers.
With its ability to harness the increased performance and reduced I/O latency available with solid state storage technology, PCI Express (PCIe) has quickly gained popularity as a storage interface for enterprise SSDs. Previously, SSDs connected to the PCIe bus on the server through a connection protocol, such as SAS or SATA. By eliminating this intermediate connection, I/O efficiency is increased and overall latency is reduced. As a result, PCIe-connected SSDs in a traditional 2.5" storage form factor, as defined by the SSD form factor Work Group Specification, are paving the way for new protocols to be developed specifically for SSD devices.
Two controller interface standards have emerged that define the protocol for communicating to PCIe-based enterprise SSDs: NVM Express (NVMe) and SCSI Express (SCSIe). Both protocols provide a highly efficient command set that allows the host server to communicate faster with the SSD.
Both NVM Express and SCSI Express optimize the command structure to allow the data packet for a single 4KB command to be fully contained in a 64-byte structure that correlates to the size of a cache line in current computer systems. The queuing layers allow up to 64K queues that are each 64K commands deep and enable today's highly parallel hosts (multi-core, multi-processor) to issue IOPS at high rates to the PCIe SSD devices, which in turn can consume them at the same rate. Interrupts are coalesced to further reduce the per I/O compute requirement, freeing up the server to spend more compute power on processing, such as transaction processing or server virtualizations; or enabling scalability, including adding more devices per server or increasing the I/O capacity of a single box.
NVM Express and SCSI Express took slightly different approaches when defining their interfaces, with the same goal, of unlocking the parallelism of today's hosts and providing a low latency path to storage. Aware of the bottlenecks in traditional storage protocols, the NVMe architects took a "clean sheet of paper" approach and developed a highly efficient command protocol which an SSD uses to connect to a processor subsystem. SCSI Express designers focused equally on low latency, but also worked on maintaining the SCSI management commands and infrastructure.
On the mechanical side of the standards picture, PCIe-connected SSDs had typically been available only in card form factors that were plugged into the back of the server. In order to make PCIe-based SSDs standard in the high availability enterprise server environment, a front-side standard 2.5" form factor was necessary. The SSD Form Factor Work Group created a 2.5" form factor standard, and a new connector standard, which became SFF-8639. These device and mechanical specifications will allow device and server manufacturers to design new standalone products around a common specification such that any new standalone PCIe SSD will connect seamlessly to the front of the server box.
The PCIe SSD is a new and compelling entrant on the enterprise storage stage. Previously, this market was characterized by proprietary implementations. Nowadays, new mechanical standards, such as SFF-8639, and new controller interface standards, such as NVM Express and SCSI Express, are enabling a plug-and-play market to emerge around 2.5" PCIe-based SSDs. This is resulting in continuity of supply and other "wins" across the board, including the ability to amortize development costs across several customers and more suppliers offering the same device. These new standards will offer immediate performance benefits that can be leveraged by existing software applications, caching solutions and operating systems to unleash the performance benefits of PCIe-connected SSDs, while acting as a bridge technology for future capabilities.
By Mike James, Director, Engineering, SanDisk
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