htdocs in RAM disk?

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http://serverfault.com – I was wondering the other day why wouldn't one place all the htdocs related files in a RAM disk. It seems to me, that this would greatly improve the time it takes for the disk to lookup and read files, greatly improving performance - specially for high websites with a high concurrency hit rate. Is there a package that can implement this in a transparent way for reading operations, while at the same time making sure the write operations are safely stored on disk? If not, is there a reason not to? (HowTos)