I was curious about this myself and did some research a while back.
I actually found, of course I cannot find it again, an benchmark done by MIT I think it was, which showed that retreival times were actually faster than a native Linux file system ext3. Of course the RDBMS wasn't indicated, but when the two were compared, DB was actually faster.
I'm currently reading up on the *nix kernel and I am somewhat convinced that using a DB to retreive might very well be faster, as an optimized DB with indexing, etc is (from what I can tell as of now) less work than a file system. I'm not sure if file indexing or fetching is handled by the VFS (I imagine it is) but that is a complicated layer of abstraction which would undoughtedly(sp???) slow things down a bit.
Of course this depends on the FS used. I'm sure you could customize the FS to serve files equally as fast, which is what Google did I think,
http://en.wikipedia.org/wiki/Google_File_System
On the typical web server system however, I would argue that because of the VFS layer and additional ext3 overhead (which I assume most Linux distros will start using by default - if not already???) it's likely faster to use a DB engine.
Ultimately I think it depends on how you view what a RDBMS is. Personally, for long time, MySQL was a system used to store linear, albeit related *records* of information, not sure if I can accept MySQL for anything more (ie: file storage system).
M$ is heading in that direction with their own (was supposed to be realesed with Vista - but I don't think it is) SQL storage file system...so that says something I guess.
DB file storage on the web has a serious problem though. That is, every file retreived would need to open a *new* connection to the database. Opening a connection is over head which should be avoided, but impossible to do so without caching the files on the native file system, in which case, why store the files on a DB in the first place?
Really, I think this is a matter of opinion and personal taste, as both sides have valid arguments...
Cheers
