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Compression Types

There are several different ways that files can be compressed on the hard disk, in terms of the logical mechanisms for performing the compression and decompression. (There are also many different compression algorithms that can be used to perform the compression, but the details of how the compression is actually done are hidden entirely from the user.) Which of these methods you choose depends entirely on the nature of the system you are using, and your compression needs of course.

The most common compression methods used on PCs are as follows:

• Utility-Based File Compression: A very popular form of disk compression, used by virtually every PC user whether they realize it or not, is file-by-file compression using a compression utility. With this type of compression, a specific utility program is used to compress one or more files into a compressed file (often called an archive), and is also used to decompress the compressed file (in some cases two complementary programs are used). The operating system knows nothing about the compression; to it the compressed file is just another file. In order to use the compressed file at all, it must be decompressed. The most popular compression system of this sort is the wildly popular PKZIP package, and its derivatives such as WinZip. Virtually all software or large files that you download from the Internet for example, use some form of this compression.
• Operating System File Compression: While not supported by the FAT file system used by DOS and most versions of Windows, some operating systems support the compression of files on an individual basis within the operating system itself. For example, Windows NT and Windows 2000 support this feature when you use the NTFS file system. This is in many ways the best type of compression, because it is both automatic (decompression is done by the operating system when the file is needed by any program) and it allows full control over which types of files are compressed. See here for more on NTFS compression.
• Volume Compression: This option is distinctly different from compressing individual files. Using the appropriate operating system, it is also possible to create entire disk volumes that are compressed. This has traditionally been done either through utilities provided with the operating system, or through third-party packages. Volume compression allows you to save disk space without having to individually compress files to use them. Every file that is copied to the compressed volume is automatically compressed, and each file is automatically decompressed when any software program needs it. Volume compression is transparent to the use and generally works well on most PCs. As mentioned in the introduction to this section, it is not used much on newer machines any more, because disks today are so large and cheap.

Of these types of compression, utility-based file compression is the most commonly used. It is relatively straight-forward; you use a program to create a compressed file and another to look at it. From the operating system's perspective, the compressed files and the utilities that use it are just like any other files and programs on the disk, no different than say, a word processor and a word processing document file. Newer utilities and operating systems can actually let you access the files contained within compressed files without decompressing them! See this page for more on file-based compression products.

Volume compression, on the other hand, is less commonly used today, though it was once quite popular. It has more complicating factors involved in its usage. In particular, there are performance considerations and safety and compatibility issues that need to be carefully weighed before using volume compression. Several other pages in this section also discuss various features of volume compression.

Next: File Compression Products

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