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A PCIe SSD (PCIe solid-state drive) is a high-speed expansion card that attaches a computer to its peripherals. PCIe, which stands for Peripheral Component Interconnect Express, is a serial expansion bus standard. PCIe slots can have different sizes, based on the number of bidirectional lanes that connect to it.
PCIe SSD storage is an alternative to a server-side flash deployment. PCIe serves as the underlying transport layer for data on graphics cards or add-in cards. PCIe SSDs integrate flash directly on a server motherboard. Each PCIe device connects to the host through its own serial link, eliminating the need to share a bus. The point-to-point architecture lowers latency and boosts data transfer rates between a server and storage.
PCIe form factors and use cases
Format specifications for PCIe-based devices are developed and maintained by the PCI Special Interest Group (PCI-SIG). The current version, PCIe 3.0, was released in November 2010. PCIe 4.0 is slated for release in 2017.
Support is emerging for PCIe SSDs designed for the nonvolatile memory express (NVMe) specification. The NVMe protocol provides an optimized command set for accessing a PCIe SSD. NVMe exploits the parallelism of PCIe 3.0 to accelerate performance.
The PCI-SIG also authored standards for M.2 SSDs, a small form factor designed for internally mounted expansion cards. The M.2 specification replaced the mSATA form factor. M.2 SSDs are equipped with four lanes of PCIe 3.0 bandwidth.
Prime PCIe flash use cases include applications with read and write access measured in tens of microseconds. This reduced latency means faster response times for processing the large workloads associated with online transaction processing and data warehousing. Data centers use PCIe flash to meet the intensive input/output (I/O) requirements of these workloads.
PCIe SSD vs. SATA SSDs
As previously discussed, PCIe-attached SSDs avoid the bottlenecks associated with SATA or SAS-attached SSDs. The number of PCIe lanes per SSD determines the speed of data transfer. A 16-lane device built on the PCIe 3.0 specification can support approximately 32 gigabytes per second. By contrast, SSDs built with a SATA III controller provide a maximum transfer rate of about 600 megabytes per second.
PCIe drive vendors have moved only gradually to implement the 3.0 specification. Most vendors are still shipping products based on PCIe 2.0.
The SATA v3.2 specification defines SATA Express connectors for host and device connectors that simultaneously support SATA and PCIe protocols.
PCIe SSD drawbacks
The PCIe multipurpose bus carries varied data to the processor. Despite its inherent performance benefits, PCIe SSDs have a higher cost per gigabyte than traditional SSDs.
The lack of standard storage commands is another drawback. PCIe SSD device makers are required to write and qualify a custom software driver for operating systems.
The future of PCIe flash
For the foreseeable future, PCIe flash figures to coexist in enterprise data centers with traditional SSDs and nonvolatile memory technologies.