All you ever wanted to know about hard drives but never dared ask

Heavy Rotation

Introduced by IBM in the 1950s, hard drives rapidly became the primary permanent storage technology of the computer industry and have retained this primacy for the past 50 years. Despite the mounting challenge posed by solid-state drives, the world's hunger for storage space continues to grow unabated, and the relative cost and performance of HDDs and SSDs are not expected to change in the medium term, making an understanding of this storage technology a key instrument of performance tuning.

The primary observation from which to start is that modern CPUs operate at speeds that make a spinning disk drive resemble the race between a rocket and a turtle: Whereas the CPU can access data in the L1 cache at effectively zero latency, RAM can as be much as 20 times slower than the L2 cache, which is itself five times as slow as its L1 counterpart [1]; this is the von Neumann bottleneck .

Spinning disk media come in at 13,700 times the latency of the L1 cache. Although hard disks remain impressive achievements of mechanical engineering, their moving parts simply cannot keep up with electrons, and the best performance decision is the one that avoids critical-path access to permanent storage altogether [2]. An unprintable infographic [3] makes this point very poignantly (be sure to zoom in at the very top of the picture); optimization is simply a race to get to wait on I/O faster if permanent storage access is not architected properly.

Disks store data in concentric tracks recorded using a constant angular velocity scheme referred to as ZCAV [4]. Unlike optical media, which use constant linear velocity, hard disks do not need to alter speed as they access different parts of the platter.

This approach has interesting

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