Understanding Storage Delays

Jim PappasJim Pappas of Intel, a fellow member of SNIA (the Storage Networking Industry Association) shared a really intuitive way to understand storage delays at the last Storage Developer Conference (SDC).  It’s very simple.  First consider these two facts:

  • The difference between the speed of system memory and that of a hard disk drive (HDD) is roughly 6 orders of magnitude, or 1 million times
  • SSDs split the gap.  An SSD is about 1,000 times faster than an HDD, and is about 1,000 times slower than system memory.  Memory access times are measured in nanoseconds (ns), SSDs in microseconds (µs) and HDDs in milliseconds (ms)

The problem with understanding this (ns, µs, ms) is that to the most people these measurements are all so tiny that they appear to be about the same.

To put things in a more human perspective lets slow everything down by about 1 billion times:

  • System memory access is now the same as a human heartbeat… Every heartbeat you get another piece of data
  • An SSD access is about the time it takes you to walk a mile
  • An HDD access is the amount of time that it would take you to bicycle from San Francisco to Miami

In these basic terms it is no wonder that everyone’s excited about SSDs.  Now you can walk to the neighborhood store for a carton of milk rather than bicycle across an entire continent.  Needless to say, the result is that you can get a lot more work done significantly faster.

It should then come as no surprise that I constantly repeat that SSDs should not be viewed as a replacement for HDDs, but as an element to be used sparingly in a system to accelerate performance.  SSDs will always be much faster and much more expensive than HDDs.  There will be a place for both technologies in computing systems for the foreseeable future.


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