what you need is http://code.google.com/p/pagecache-mangagement/
This tool allows the user to limit the amount of pagecache used by applications under Linux. This is similar to nice, ionice etc. in that it usually doesn't make an application go faster, but does reduce the impact of the application on other applications performance. This is especially useful for applications that walk sequentially through data sets larger than memory, as discarding their pagecache does not reduce their performance (although this tool does add overhead of about 2%).
Although it is little more than a proof-of-concept it seems to be fairly
useful. When running
pagecache-management.sh dd if=100-mb-file of=foo
pagecache-management.sh cp -a/usr/src/linux-2.6.20/usr/src/foo
DRM-free writes: The events of the past two weeks have clarified the BBC’s stance on allowing interoperability with open-source iPlayer clients. I have therefore decided to withdraw get_iplayer with immediate effect.