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April 28, 2008
Scaling Internet Datacenters for Web 2.0 Applications
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The World Wide Web continues to grow in both breadth and depth as more users and more applications go online. The changes over the past few years have been so profound that the industry has adopted the term "Web 2.0" to portray the significance of newer applications like blogs, social networking, podcasts, software as a service (SaaS), RSS feeds, Wikis, and many other forms of many-to-many publishing and collaboration. HTML is now almost old-fashioned as it gives way to languages like the Extensible Markup Language and Java, and their combination, AJAX (Asynchronous JavaScript and XML), designed to bring unprecedented power to the browser.
The impact on the Internet datacenter is equally profound. Gone are the isolated silos of Web servers simply serving up content. Today's Web applications require continual, real-time interactions among multiple client-facing and backend servers. Content must be managed dynamically. Computing is becoming virtualized and distributed, potentially in clusters, grids or clouds. Storage is becoming tiered and consolidated. The entire Internet datacenter must now operate more seamlessly to facilitate Web 2.0 applications. The biggest challenge faced in today's biggest datacenters is making such seamless networking both scalable and affordable.
Scaling the Datacenter -- Past and Present
Datacenters scale today with various combinations of three different networking technologies: Layer 2 Ethernet for edge or access aggregation; Layer 3 IP for distribution switching, core and border routing; and Fibre Channel for storage area networks (SANs). While this approach does indeed scale, it is both complex and costly.
The largest datacenters are now encountering another problem: despite server virtualization and storage consolidation initiatives, they are simply running out of space, power and cooling capacity, owing in large part to the complexity of the myriad interconnects involved.
The diagram below shows how the servers in today's typical datacenter are organized into units, and how these multiple units scale. In this example, Ethernet switching is employed in the Access Layer, with IP routing in the Distribution and Core Layers, as well as at the Border. Each scalable unit is limited to a maximum of 640 servers in 16 racks with up to 40 servers per rack. Each server connects via Gigabit Ethernet (GE) to an Access Layer "top-of-rack" switch with 10 GE connections to the Distribution Layer router. Each 10 GE Distribution Layer router utilizes a total of 28 ports connected as follows: 16 ports for the 16 top-of-rack switches; 4 ports (as a Link Aggregation Group or LAG) connecting with the companion Distribution switch; and 8 ports to the Core. The Core routers also utilize a total of 28 ports each as follows: 12 ports serving 2 connections each from the 6 scalable units; 8 ports (for two 4-port LAGs) connecting with the companion Core switch; and 8 ports to the Border.
Most datacenters today employ a three-layered architecture servers -- Access, Distribution and Core -- to interconnect with the servers organized into scalable units. In this example there are six such scalable units supporting a maximum of 3,840 servers.
In the above example, using a popular switch/router with a maximum non-blocking capacity of 32 10GbE ports, but with a single 4-port card slot being consumed by a redundant supervisor card, the configuration can support a maximum of 3,840 servers. Note that these servers are over-subscribed 4:1 because there is only 20 Gbps of capacity for each rack of 40 servers, and only 8 Core Layer ports going upstream to service the 16 Access Layer ports going downstream.
Something better clearly is needed to support -- preferably more cost-effectively -- the orders of magnitude more servers now being deployed in some of the industry's larger datacenters.
That something better could be something that is also quite familiar: Ethernet. A scalable Layer 2 Ethernet Distribution Layer also overcomes the incumbent Layer 3 approach's two weaknesses with its plug-and-play simplicity and industry-leading affordability. But can Ethernet really meet the demands of today's large and growing datacenters? Can Ethernet really deliver the scalability provided by Layer 3 routing while preserving its renowned simplicity and affordability?
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