Page 385 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes read-only file systems for CD-ROMs and DVDs. It is possible (even common) to have the same
running system have access to multiple file system types available at the same time.
In this unit, we will treat the NTFS file system because it is a modern file system unencumbered
by the need to be fully compatible with the MS-DOS file system, which was based on the CP/M
file system designed for 8-inch floppy disks more than 20 years ago. Times have changed and
8-inch floppy disks are not quite state of the art any more. Neither are their file systems. Also,
NTFS differs both in user interface and implementation in a number of ways from the UNIX file
system, which makes it a good second example to study. NTFS is a large and complex system
and space limitations prevent us from covering all of its features, but the material presented
below should give a reasonable impression of it.
13.2.1 Fundamental Concepts of Windows 2000 File System

Individual file names in NTFS are limited to 255 characters; full paths are limited to 32,767
characters. File names are in Unicode, allowing people in countries not using the Latin alphabet
(e.g., Greece, Japan, India, Russia, and Israel) to write file names in their native language. For
example, τλ∈ is a perfectly legal file name. NTFS fully supports case sensitive names (so foo is
different from Foo and FOO). Unfortunately, the Win32 API does not fully support case-sensitivity
for file names and not at all for directory names, so this advantage is lost to programs restricted
to using Win32 (e.g., for Windows 98 compatibility).
An NTFS file is not just a linear sequence of bytes, as FAT-32 and UNIX files are. Instead, a
file consists of multiple attributes, each of which is represented by a stream of bytes. Most files
have a few short streams, such as the name of the file and its 64-bit object ID, plus one long
(unnamed) stream with the data. However, a file can also have two or more (long) data streams
as well. Each stream has a name consisting of the file name, a colon, and the stream name, as in
foo:stream1. Each stream has its own size and is lockable independently of all the other streams.
The idea of multiple streams in a file was borrowed from the Apple Macintosh, in which files
have two streams, the data fork and the resource fork. This concept was incorporated into NTFS
to allow an NTFS server be able to serve Macintosh clients.

File streams can be used for purposes other than Macintosh compatibility. For example, a photo
editing program could use the unnamed stream for the main image and a named stream for a
small thumbnail version. This scheme is simpler than the traditional way of putting them in the
same file one after another. Another use of streams is in word processing. These programs often
make two versions of a document, a temporary one for use during editing and a final one when
the user is done. By making the temporary one a named stream and the final one the unnamed
stream, both versions automatically share a file name, security information, timestamps, etc.
with no extra work.
The maximum stream length is 2 bytes. To get some idea of how big a 2 -byte stream is, imagine
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that the stream were written out in binary, with each of the 0s and 1s in each byte occupying
1 mm of space. The 2 -mm listing would be 15 light-years long, reaching far beyond the solar
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system, to Alpha Centauri and back. File pointers are used to keep track of where a process
is in each stream, and these are 64 bits wide to handle the maximum length stream, which is
about 18.4 exabytes.

The Win32 API function calls for file and directory manipulation are roughly similar to their
UNIX counterparts, except most have more parameters and the security model is different.
Opening a file returns a handle, which is then used for reading and writing the file. For graphical
applications, no file handles are predefined. Standard input, standard output, and standard error
have to be acquired explicitly if needed; in console mode they are preopened, however, Win32
also has a number of additional calls not present in UNIX.

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