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PCIe vs ethernet: A face-off or coexistence?

Posted: 10 Apr 2014     Print Version  Bookmark and Share

Keywords:PCI Express  PCIe  Ethernet  Fibre Channel  SR-IOV 

Additional advantages of shared I/O are:
 • As I/O speeds increase, the only additional investment needed is to change the I/O adapter cards. In earlier deployments, when multiple I/O technologies existed on the same card, designers would have to re-design the entire system, whereas in the shared-I/O model, they can simply replace an existing card with a new one when an upgrade is needed for one particular I/O technology.
 • Since multiple I/O end-points don't need to exist on the same cards, designers can either manufacture smaller cards to further reduce cost and power, or choose to retain the existing form factor and differentiate their products by adding multiple CPUs, memory and/or other end-points in the space saved by eliminating multiple I/O end-points from the card.
 • Designers can reduce the number of cables that crisscross a system. With multiple interconnect technologies comes the need for different (and multiple) cables to enable bandwidth and overhead protocol. However, with the simplification of the design and the range of I/O interconnect technologies, the number of cables needed for proper functioning of the system also are reduced, thereby eliminating the complexity of the design and delivering cost savings.

Implementing shared I/O in a PCIe switch is the key enabler to architectures depicted in figure 2. As mentioned earlier, MR-IOV technology hasn't quite taken off and a prevailing opinion is that it probably never will. To the rescue comes Single-Root I/O Virtualisation (SR-IOV) technology, which implements I/O virtualisation in the hardware for improved performance, and makes use of hardware-based security and quality-of -service (QoS) features in a single physical server. SR-IOV also allows the sharing of an I/O device by multiple guest operating system (OSes) running on the same server.

In 2007, the PCI-SIG released the SR-IOV specification that calls for one physical PCIe device—be it a network interface card, host bus adapter or host channel adapter—to be divided into multiple virtual functions. Each virtual function can then be used by a virtual machine, allowing one physical device to be shared by many virtual machines and their guest OSes.

This requires I/O vendors to develop devices that support SR-IOV that provide the simplest approach to sharing resources or I/O devices among different applications. The trend has been that most of the end-point vendors are supporting SR-IOV and many more will continue to support this requirement.

Adding to its many advantages already cited here, PCIe is also a lossless fabric at the transport layer.

The PCIe specification has defined a robust flow-control mechanism, which prevents packets from being dropped. Every PCIe packet is acknowledged at every hop, insuring a successful transmission. In the event of a transmission error, the packet is replayed again – something that occurs in hardware, without any involvement of upper layers. Data loss and corruption in PCIe-based storage systems, therefore, are highly unlikely.

PCIe offers a simplified solution by allowing all I/O adapters (10GbE or FC or others) to be moved outside the server. With a PCIe switch fabric providing virtualisation support, each adapter can be shared across multiple servers and at the same time provide each server with a logical adapter. The servers (or the virtual machines on each server) continue to have direct access to their own set of hardware resources on the shared adapter. The resulting virtualisation allows for better scalability wherein the I/O and the servers can be scaled independently of each other. I/O virtualisation avoids over-provisioning the servers or the I/O resources, thus leading to cost and power reduction.

Table 1 provides a high-level overview of the cost comparison and table 2 provides the high-level overview of the power comparison when using PCIe over 10G Ethernet.

Table 1: Cost savings comparison between PCIe and Ethernet.

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