For example, I have a db with 20 GB of data and only 2 GB of RAM, the swap is disabled. Can I find and paste data? How bad would that be?
This is best done on google, but many sources say that when your working set exceeds your RAM size, performance will decrease significantly.
Sharding can be an interesting option, rather than adding more RAM.
http://www.mongodb.org/display/DOCS/Checking+Server+Memory+Usage
http://highscalability.com/blog/2011/9/13/must-see-5-steps-to-scaling-mongodb-or-any-db-in-8-minutes.html
http://blog.boxedice.com/2010/12/13/mongodb-monitoring-keep-in-it-ram/
http://groups.google.com/group/mongodb-user/browse_thread/thread/37f80ff39258e6f4
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, 4square :
http://highscalability.com/blog/2010/10/15/troubles-with-sharding-what-can-we-learn-from-the-foursquare.html
http://groups.google.com/group/mongodb-user/browse_thread/thread/528a94f287e9d77e
http://blog.foursquare.com/2010/10/05/so-that-was-a-bummer/
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, : http://www.colinhowe.co.uk/2011/02/23/mongodb-performance-for-data-bigger-than-memor/