Information Technology Reference
In-Depth Information
result of these needs, many individuals own 1 TB or more of storage for
their personal files. This is an enormous amount: if you printed 1 TB of
data as text on paper, you would produce a stack about 20 miles high.
In contrast, for less than $100 you can buy 1 TB of storage that fits in a
shoebox.
High performance. For programs to use data, they must be able to
access it, and for programs to use large amounts of data, this access must
be fast. For example, users want program start-up to be nearly instan-
taneous, a business may need to process hundreds or thousands of orders
per second, or a server may need to stream a large number of video files
to different users.
Named data. Because users store a large amount of data, because some
data must last longer than the process that creates it, and because data
must be shared across programs, storage systems must provide ways to
easily identify data of interest. For example, if you can name a file (e.g.,
/home/alice/assignments/hw1.txt
) you can nd the data you want out
of the millions of blocks on your disk, you can still find it after you shut
down your text editor, and you can use your email program to send the
data produced by the text editor to another user.
Controlled sharing. Users need to be able to share stored data, but
this sharing needs to be controlled. As one example, you may want to
create a design document that everyone in your group can read and write,
that people in your department can read but not write, and that people
outside of your department cannot access at all. As another example, it is
useful for a system to be able to allow anyone to execute a program while
only allowing the system administrator to change the program.
Nonvolatile storage and le systems. The contents of a system's main
DRAM memory can be lost if there is an operating system crash or power failure.
In contrast, nonvolatile storage is durable and retains its state across crashes and
Denition: nonvolatile
storage
power outages; nonvolatile storage is also called or persistent storage or stable
Definition: persistent
storage
storage. Nonvolatile storage can also have much higher capacity and lower cost
than the volatile DRAM that forms the bulk of most system's \main memory."
Definition: stable storage
However, nonvolatile storage technologies have their own limitations. For
example, current nonvolatile storage technologies such as magnetic disks and
high-density flash storage do not allow random access to individual words of
storage; instead, access must be done in more coarse-grained units|512, 2048,
or more bytes at a time.
Furthermore, these accesses can be much slower than access to DRAM; for
example, reading a sector from a magnetic disk may require activating a motor
to move a disk arm to a desired track on disk and then waiting for the spin-
ning disk to bring the desired data under the disk head. Because disk accesses
