Information Technology Reference
In-Depth Information
Metric
Spinning Disk
Flash
Capacity/Cost
Excellent
Good
Sequential BW/Cost
Good
Good
Random I/O per Second/Cost
Poor
Good
Power Consumption
Fair
Good
Physical Size
Good
Excellent
Figure12.8: Relative advantages and disadvantages of spinning disk and flash
storage.
in 2011 dollars): the cost per byte has improved by a factor of about 400,000
over 27 years|over 50% per year for nearly 3 decades.
Recent rates of improvement for flash storage have been even faster. For
example, in 2001, the Adtron S35PC 14 GB ash drive cost $42,000. Today's
Intel 320 costs 70 times less for 21 times more capacity, an improvement of
about 2x per year over the past decade.
Similar capacity improvements for spinning disk and flash are expected for
at least the next few years. Beyond that, there is concern that we will be
approaching the physical limits of both magnetic disk and flash storage, so the
longer-term future is less certain. (That said, people have worried that disks
were approaching their limits several times in the past, and we will not be
surprised if the magnetic disk and flash industries continue rapid improvements
for quite a few more years.)
In constrast to capacity, performance is likely to improve more slowly for
both technologies. For example, a mid-range spinning disk in 1991 might have
had a 1.3 MB/s maximum bandwidth and an 17 ms average seek time. Band-
widths have improved by about a factor of 90 in two decades (about 25% per
year) while seek times and rotational latencies have only improved by about a
factor of two (less than 4% per year.) Bandwidths have improved more quickly
than rotational latency and seek times because bandwidth benefits from increas-
ing storage densities, not just increasing rotational rates.
For SSDs, the story is similar, though recent increases in volumes have helped
speed the pace of improvements. For example, in 2006 a BitMicro E-Disk flash
drive could provide 9,500 to 11,700 random reads per second and 34-44 MB/s
sustained bandwidth. Compared to the Intel 320 SSD from 2011, bandwidths
have improved by about 40% per year and random access throughput has im-
proved by about 25% per year over the past 5 years.
New techologies. This is an exciting time for persistent storage. After
decades of undispuated reign as the dominant technology for on-line persis-
tent storage, spinning magnetic disks are being displaced flash storage in many
