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Disk compression is a technique that exploits a known characteristic of most types of data: they have repeating patterns. By using software that implements special compression and decompression algorithms, it is possible to store most files in a way that they take up less space on the disk than they normally would. Compression became very popular in the late 1980s and early 1990s, when software and data began to grow in size much faster than hard disks were. Products were created to allow for the compression of entire FAT volumes, and also files or groups of files. I discuss disk compression in general terms, including how it works and how it is implemented, in this section of the site.
One of the most useful features that is built into NTFS is file-based compression that can be used to compress individual files or folders on almost any NTFS partition, under Windows NT 3.51 or later. While Microsoft's consumer operating systems such as Windows 3.1 and Windows 9x support compression of entire disk volumes (under some circumstances), they do not let you easily decide to compress a portion of a volume. Doing that has required resorting to the use of a third-party file compression utility. Under NTFS, you can easily compress one or more files or folders by opening their properties and telling the operating system to compress them. The compression is handled by the operating system during writes, and decompression is automatic whenever an application needs to read the file.
As is the case for FAT volume compression, the performance impact of NTFS compression is a complex question. Performance when working with compressed files can be degraded compared to regular files, as a result of the overhead required to compress and decompress the file on a regular basis--it takes CPU time. On the other hand, compressing a file means that it takes up less space on the disk, which reduces the amount of time required to write to the volume or read from it, potentially counteracting the compression overhead. In general, since hard disks today are quite large, most people use NTFS compression only on infrequently-used files. For example, if you have large numbers of old database files that you aren't using, you can save a lot of space by compressing them. Of course, you might be better off archiving them, to CD-RW or other storage media. Still, most NTFS users consider compression a useful feature of the file system.
Note: NTFS compression is
not supported on NTFS volumes that use a cluster size greater than 4 kiB. This is a
primary reason why the default cluster size for NTFS
volumes is never made larger than 4 kiB under Windows NT 3.51 or later.
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