Access to data in memory is much faster than access to data on disk. It
takes a few clock cycles (perhaps 5 × 10−9 seconds) to access a byte in
memory, but much longer to transfer it from disk (about 2 × 10−8 seconds).
Consequently, we want to keep as much data as possible in memory,
especially those data that we need to access frequently. We call the technique of keeping frequently used disk data CACHING in main memory caching