Information Technology Reference
In-Depth Information
application domains, giving both operating system designers and application
writers an opportunity to reexamine how to best use storage. Looking forward,
many researchers speculate that new technologies may soon be nipping at the
heels and even surpassing flash storage.
For example phase change memory (PCM) uses a current to alter the state
Denition: phase change
memory
of chalcogenide glass between amorphous and crystaline forms, which have sig-
nificantly different electrical resistance and can therefore be used to represent
data bits. Although PCM does not yet match the density of flash, researchers
speculate that the technology is fundamentally more scalable and will ultimately
be able to provide higher storage densities at lower costs. Furthermore, PCM
is expected to have much better write performance and endurance than flash.
As another example, memristors are circuit elements whose resistance de-
Denition: memristors
pends on the amounts and directions of currents that have flowed through them
in the past. A number of different memristor constructions are being pur-
sued, and some have quite promising properties. For example, in 2010 Hewlett
Packard labs described a prototype memristor constructed of a thin titanium
dioxide film with 3 nm by 3 nm storage elements that can switch states in 1 ns.
These densities are similar to contemporary flash memory devices and these
switching times are similar to contemporary DRAM chips. The devices also
have write endurance similar to flash, and extremely long (theoretically unlim-
ited) storage lifetimes. Furthermore, researchers believe that these and others
memristors' densities will scale well in the future. For example, in 2009 a design
for 3-D stacking of memristors was published in the
Proceedings of National
Academy of Sciences
by Dmitri Strukov and R. Stanley Williams of HP Labs.
If technologies such as these pan out as hoped, operating system design-
ers will have opportunities to rethink our abstractions for both volatile and
nonvolatile storage: how should we make use of word-addressible, persistent
memory with densities exceeding current flash storage devices and with mem-
ory access times approaching those of DRAM? What could we do if each core
on a 32 core processor chip had acess to a few gigabytes of stacked memristor
memory?
Exercises
1. Discussion. Some high-end disks in the 1980s had multiple disk arm
assemblies per disk enclosure in order to allow them to achieve higher
performance. Today, high-performance server disks have a single arm
assembly per disk enclosure. Why do you think disks so seldom have
multiple disk arm assemblies today?
2. How many sectors does a track on the disk described in Figure 12.3 on
page 360 have?
