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For the company, see Google, Inc.; for the search engine see Google search; for other uses see Google (disambiguation).

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Google, being the most popular Internet search engine (over 50% market share), requires large computational resources in order to provide their service. This article describes Google's technological infrastructure, as presented in the company's public announcements.

Contents 1 Network topology 2 Main index 3 Server types 4 Server hardware and software 4.1 Original hardware 4.2 Current hardware 4.3 Project 02 5 Server operation 6 References 7 External links

[edit] Network topology

Google maintains an estimated 450,000 servers, arranged in racks located in clusters in cities around the world, with major centers in Mountain View, California; Virginia; Atlanta, Georgia; Dublin, Ireland; and a new facility constructed in 2006 in The Dalles, Oregon.<ref name="howgoogleworks">Carr, David F. "How Google Works." Baseline Magazine. July 6, 2006. Retrieved on July 10, 2006.</ref> When an attempt to connect to Google is made, Google's DNS servers perform load balancing to allow the user to access Google's content most rapidly. This is done by sending the user the IP address of a cluster that is not under heavy load, and is geographically proximate to them. Each cluster has a few thousand servers, and upon connection to a cluster further load balancing is performed by hardware in the cluster, in order to send the queries to the least loaded Web Server. This makes Google one of the biggest and most complex known Content Delivery Networks.

Racks are custom-made and contain 40 to 80 servers (20 to 40 1U servers on either side), new servers are 2U Rackmount systems.<ref name="google_arch"> Web Search for a Planet: The Google Cluster Architecture (Luiz André Barroso, Jeffrey Dean, Urs Hölzle) </ref> Each rack has a switch. Servers are connected via a 100 Mbit/s Ethernet link to the local switch. Switches are connected to core gigabit switch using one or two gigabit uplinks.^{[citation needed]}

[edit] Main index

Since queries are composed of words, an inverted index of documents is required. Such an index allows obtaining a list of documents by a query word. The index is very large due to the number of documents stored in the servers, therefore it needs to be split up into "index shards". Each shard is hosted by a set of index servers. The load balancer decides which index server to query based on the availability of each server.

[edit] Server types

Google's server infrastructure is divided in several types, each assigned to a different purpose:<ref name="google_arch"/>

• Google DNS Servers answer the DNS requests and serve as intelligent, worldwide load-balancer. They guess the data center nearest to the user to speed up all http requests.

• Google Web Servers coordinate the execution of queries sent by users, then format the result into an HTML page. The execution consists of sending queries to index servers, merging the results, computing their rank, retrieving a summary for each hit (using the document server), asking for suggestions from the spelling servers, and finally getting a list of advertisements from the ad server.

• Data-gathering servers are permanently dedicated to spidering the Web. They update the index and document databases and apply Google's algorithms to assign ranks to pages.

• Index servers each contain a set of index shards. They return a list of document IDs ("docid"), such that documents corresponding to a certain docid contain the query word. These servers need less disk space, but suffer the greatest CPU workload.

• Document servers store documents. Each document is stored on dozens of document servers. When performing a search, a document server returns a summary for the document based on query words. They can also fetch the complete document when asked. These servers need more disk space.

• Ad servers manage advertisements offered by services like AdWords and AdSense.

• Spelling servers make suggestions about the spelling of queries.

[edit] Server hardware and software

[edit] Original hardware

The original hardware (ca. 1998) that was used by Google when it was located at Stanford University, included:<ref>"Google Stanford Hardware." Stanford University (provided by Internet Archive). Retrieved on July 10, 2006.</ref>

• Sun Ultra II with dual 200MHz processors, and 256MB of RAM. This was the main machine for the original Backrub system.
• 2 x 300 MHz Dual Pentium II Servers donated by Intel, they included 512MB of RAM and 9 x 9GB hard drives between the two. It was on these that the main search ran.
• F50 IBM RS/6000 donated by IBM, included 4 processors, 512MB of memory and 8 x 9GB hard drives.
• Two additional boxes included 3 x 9GB hard drives and 6 x 4GB hard drives respectively (the original storage for Backrub). These were attached to the Sun Ultra II.
• IBM disk expansion box with another 8 x 9GB hard drives donated by IBM.
• Homemade disk box which contained 10 x 9GB SCSI hard drives.

[edit] Current hardware

Servers are commodity-class x86 PCs running customized versions of GNU/Linux. Indeed, the goal is to purchase CPU generations that offer the best performance per unit of power, not absolute performance. Other than wages, the biggest cost that Google faces is electric power consumption.^{[citation needed]} Estimates of the power required for over 450,000 servers range upwards of 20 megawatts, which could cost on the order of US$2 million per month in electricity charges.

For this reason, the Pentium II has been the most favoured processor,^{[citation needed]} but this could change in the future as processor manufacturers are increasingly limited by the power output of their devices.

Specifications:

• Over 450,000 servers<ref name="howgoogleworks"/> ranging from 533 MHz Intel Celeron to dual 1.4 GHz Intel Pentium III (as of 2005)
• One or more 80GB hard disk per server (2003)
• 2–4 GiB memory per machine (2004)

The exact size and whereabouts of the data centers Google uses are unknown, and official figures remain intentionally vague. In a 2000 estimate, Google's server farm consisted of 6000 processors, 12,000 common IDE disks (2 per machine, and one processor per machine), at four sites: two in Silicon Valley, California and two in Virginia.<ref name="architecture">Hennessy, John; Patterson, David. (2002). Computer Architecture: A Quantitative Approach. Third Edition. Morgan Kaufmann. ISBN 1-55860-596-7.</ref> Each site had an OC-48 (2488 Mbit/s) internet connection and an OC-12 (622 Mbit/s) connection to other Google sites. The connections are eventually routed down to 4 x 1 Gbit/s lines connecting up to 64 racks, each rack holding 80 machines and two ethernet switches.

[edit] Project 02

Google is currently developing a supercomputer at a data center located in the town of The Dalles, Oregon, on the Columbia River, approximately 80 miles from Portland. The project, codenamed Project 02,<ref name="project02">Markoff, John; Hansell, Saul. "Google's quasi-secret power play." San Diego Union Tribune. June 14, 2006. Retrieved on July 10, 2006.</ref> is expected to substantially add to their current global network capable of processing billions of search queries per day and a growing repertoire of other services.<ref name="project02"/> The new complex is approximately the size of two football fields with cooling towers four stories high. The project has already created hundreds of jobs in the area, mainly construction jobs at this point, with an expected 60 to 200 permanent positions later this year.<ref name="project02"/> Real estate prices in the small town of 12,000 have also increased by 40%.<ref name="project02"/>

[edit] Server operation

Most operations are read-only. When an update is required, queries are redirected to other servers, so as to simplify consistency issues. Queries are divided into sub-queries, where those sub-queries may be sent to different ducts in parallel, thus reducing the latency time.<ref name="google_arch"/>

In order to avoid the effects of unavoidable hardware failure, data stored in the servers may be mirrored using hardware RAID. Software is also designed to be fault tolerant. Thus when a system goes down, data is still available on other servers, which increases the reliability.<ref name="google_arch"/>

[edit] References

[edit] External links

• Google Research Publications
• Google Supercomputer
• The Google Linux Cluster — Video about Google's Linux cluster
• Web Search for a Planet: The Google Cluster Architecture (Luiz André Barroso, Jeffrey Dean, Urs Hölzle)
• How Google Works
• Original Google Hardware Pictures