Information Technology Reference
In-Depth Information
Figure12.7: In 2011, ash storage \keys" such as this one can store as much
as 256 GB in a device that is a few centimeters long, and 1-2 cm wide and tall.
MB/s per dollar) while the same Intel SSD has a read/write bandwidth of
270/205 MB/s (about 0.4 MB/s per dollar.)
Flash storage is often used when good random access performance or low
power consumption is the goal. For example, flash storage is frequently used in
database transaction processing servers, in smart phones, and in laptops. For
example, the Seagate described above drive rotates at 5900 RPM, so it takes
about 5 ms for a half rotation; even with good scheduling and even if data is
confined to a subset of tracks, it would be hard to get more than 200 random
I/Os per second from this drive (about 2.5 random I/Os per second per dollar.)
Conversely, the Intel SSD can sustain 23,000 random writes and 39,500 random
reads per second (about 38 or 66 random writes or reads per second per dollar.)
With respect to power, spinning disks typically consume 10-20W depending
on whether it is just spinning or actively reading and writing data, while a flash
drive might consume 0.5W-1W when idle and 3-5W when being accessed. Flash
drives' power advantage makes them attractive for portable applications such
as laptop and smartphone storage.
Flash memory can also have a significant form factor advantage with respect
to physical size and weight. Although some flash drives are designed as drop-in
replacements for spinning disks and so are similar in size, flash storage can be
much smaller than a typical spinning disk. For example, in 2011, a USB flash
storage \key" such as the one in Figure 12.7 can store as much as 256 GB in a
device that is not much larger than a house key.
Figure 12.8 summarizes these advantages and disadvantages; of course,
many systems need to do well on multiple metrics, so system designers may
need to compromise on some metrics or use combinations of technologies.
Technology trends. Over the past decades, the cost of storage capacity has
fallen rapidly for both spinning disks and solid state storage. Compare the 2 TB
disk drive for $80 in 2011 to a 15 MB drive costing $113 in 1984 (or about $246
