Access times This disk access time is for random I/O Once the first bit is read, the rest (sequential I/O) is almost free (only 0.02 ms) Ratio of random I/O to sequential I/O is, therefore, (12 / 0.02) 600 times Bulk transfer rates are calculated more precisely using gaps Time to access a byte 60 ns for DRAM (used for main memory) and 4 ns for SRAM (used for cache memory) Ratio of disk access time to memory access time 40000 for SRAM and 2500 for DRAM Disk access time is dominated by seek time and rotational latency Sequential access algorithms exploit the (almost) free access time of later bits heavily Most algorithms aim to avoid random I/Os