4.2 Blocked sort-based indexing 69
Block sizes of 8, 16, 32, and 64 kilobytes (KB) are common. We call the part
BUFFER of main memory where a block being read or written is stored a buffer.
• Data transfers from disk to memory are handled by the system bus, not by
the processor. This means that the processor is available to process data
during disk I/O. We can exploit this fact to speed up data transfers by
storing compressed data on disk. Assuming an efficient decompression
algorithm, the total time of reading and then decompressing compressed
data is usually less than reading uncompressed data.
• Servers used in IR systems typically have several gigabytes (GB) of main
memory, sometimes tens of GB. Available disk space is several orders of
magnitude larger