How much control does a block driver have over the buffer cache?

[Neil Baylis]

I'm trying to clarify my picture of how the buffer cache works. I notice
that if I do the following:

dd if=/dev/sda7 of=/dev/null bs=10k count=100

The first time will be slow, and the second and subsequent times will be
very fast because the data are now in the buffer cache.

However, this is only true if there's a filesystem mounted on the device.
This leads me to think of the buffer cache as associated with the filesystem
rather than the device.

For example, would it be possible to create a block driver that does not use
the buffer cache? I think the answer must be 'no' because the caching
happens before the block driver gets any requests to move data.
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