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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