Using deep buffers to avoid traffic spikes in the data center

This vendor-written tech primer has been edited by Network World to eliminate product promotion, nevertheless readers should note it will likely favor the submitter's approach.

With most virtualized servers supporting between two and 10 applications, the network traffic profile is changing. Not only is traffic increasing, nevertheless the potential for very large traffic spikes has increased as so then. With each of these virtualized servers, each rack now hosts between 160 and 400 application instances.

Traditional data centers did not experience network spikes because physical servers were heavily underutilized. Most servers only supported a single application and were sized to support peak needs, meaning the rest of the time they were only utilized at 10% to 20%.

However, with virtualization and cloud computing, servers are more highly utilized and more concentrated in fewer locations, and as a result, data center network traffic is changing.

As the number of applications generating network traffic increases, the aggregated result is a higher level of traffic with fewer peaks and valleys because the network peaks and valleys from each application tend to cancel each other out. But, when a majority of the applications generate a burst of traffic together, large network spikes occur.

Minute or two

While normal spikes last for seconds or even a minute or two and result in a hiccup to systems and data center operation, in high-density application environments the cascading effect of a network traffic spike impact could last for several hours and impact business operations.

For traditional network traffic, dropped packets are simply retransmitted, resulting in slower network performance. But, for storage networks, as is the case with iSCSI, dropped packets could result in a retransmit of a large block of data, increasing the stress on the network. The emerging trend toward consolidating server and storage traffic on the same network using protocols just as iSCSI and Fibre Channel over Ethernet makes it imperative that the network be immune to packet loss. Unfortunately, more complete standards like Data Center Bridging are focused on 10G Ethernet during the majority of data centers are nevertheless running 1G from the servers to the network.

As highly virtualized data centers begin to show signs of traffic spike activity, IT architects will have to focus on the switching infrastructure to ensure it is capable of handling significant spikes. It would be costly to model the tipping point when "x" number of virtual application instances running on physical servers with "y" number of cores and "z" number of 1G interfaces making "bursty" network request at p% of the time of file block sizes "q" from "r" number of files or databases at a given point in the course of time.

A typical top-of-rack switch has 8-16M bytes of buffering capacity. When this capacity is overwhelmed, packet loss occurs. Conventional wisdom to avoid packet loss is to a) look at ways to "throttle" the traffic by identifying those "offending" application instances and "tame" them or move those "heavy" applications to a separate network/rack, b) throw more network bandwidth at the problem, say by adding another 1G connection or an moreover expensive approach of upgrading the whole infrastructure to 10G, or c) take the Cadillac approach of implementing 10G infrastructure with converged network adapters and FCoE, etc.

As IT architects seek to flatten their networks in order to reduce latency within virtualized environments, they should plan to eliminate legacy switches that can be swamped with traffic spikes, and to implement switches with ample, dynamically adjustable buffers that can handle whatever the server and storage infrastructure require.

Force10 Networks develops high-performance data center solutions powered by the industry's most innovative line of open, standards-based, networking hardware and software. The company's Open Cloud Networking framework grants Web 2.0/portal operators, cloud and hosting providers, enterprise and special-purpose data center clients new levels of flexibility, performance, scale and automation -- fundamentally changing the economics of data center networking. For more information, visit www.force10networks.com.