The Evolution of File Systems
Years ago, in the days before disk operating systems, each computer was built
to run a single, proprietary application, which had complete and exclusive
control of the entire machine. The application would write its persistent data
directly to a disk, or drum, by sending commands directly to the disk controller.
The application was responsible for managing the absolute locations of data on
the disk, making sure that it was not overwriting data that was already there,
but since the application was the only one running on the machine, this task was
not too difficult.
The advent of computer systems that could run more than one application
required some sort of mechanism to make sure that applications did not write over
each other's data. Application developers addressed this problem by adopting a
single standard for marking which disk sectors were in use and which were free. In
time, these standards coalesced to become a disk operating system, which
provided various services to the applications, including a file system for managing
persistent storage. With the advent of a file system, applications no longer
had to deal directly with the physical storage medium. Instead, they simply told
the file system to write blocks of data to the disk and let the file system
worry about how to do it. In addition, the file system allowed applications to
create data hierarchies through the abstraction known as a directory. A directory
could contain not only files but other directories. which in turn could contain
their own files and directories, and so on.
The file system provided a single level of indirection between applications
and the disk, and the result was that every application saw a file as a single
contiguous stream of bytes on the disk even though the file system was actually
storing the file in discontiguous sectors. The indirection freed the
applications from having to care about the absolute position of data on a storage device.
Today, virtually all system APIs for file input and output provide
applications with some way to write information into a flat file that applications see as
a single stream of bytes that can grow as large as necessary until the disk is
full. For a long time these APIs have been sufficient for applications to store
their persistent information. Applications have made significant innovations
in how they deal with a single stream of information to provide features like
incremental "fast" saves.
In a world of component objects, however, storing data in a single flat file
is no longer efficient. Just as file systems arose out of the need for multiple
applications to share the same storage medium, so now, component objects
require a system that allows them to share storage within the conceptual framework of
a single file. Even though it is possible to store the separate objects using
conventional flat file storage, should one of the objects increase in size, or
should you simple add another object, it becomes necessary to load the entire
file into memory, insert the new object, and then save the whole file. Such a
process can be extremely time-consuming.
The solution provided by OLE is to implement a second level of indirection: a
file system within
a file. Instead of requiring that a large contiguous sequence of bytes on the
disk be manipulated through a single file handle with a single seek pointer,
OLE structured storage defines how to treat a single file system entity as a
structured collection of two types of objects
storages and streams
that act like directories and files.
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