NTFS Compression

Windows NT supports compression on an individual file basis for NTFS volumes. Files that are compressed on an NTFS volume can be read and written without first being uncompressed by another program. Uncompression happens automatically during the read of the file. The file is compressed again when it is closed or explicitly saved.

Compressed files and folders have an attribute of C when viewed in My Computer or Windows NT Explorer. You can select an alternate color for compressed files and folders by:

• On the View menu, click Options.

• On the View tab, check Display compressed files and folders with alternate color.

Only the NTFS file system can read the compressed form of the data. When an program like Microsoft Word for Windows or an operating system command like Copy requests access to the file, the NTFS file system uncompresses the file before making it available. For example, if you copy a compressed file from a another Windows NT computer to a compressed folder on your hard disk, the file will be uncompressed, copied, and recompressed.

This compression algorithm is similar to that provided by the MS-DOS 6.0 DoubleSpace® and MS-DOS 6.22 DriveSpace® compression, with one important difference — the MS-DOS functionality compresses the entire primary partition or logical drive, while the NTFS file system enables the user to compress individual files and folders in the NTFS volume.

NTFS compression is not supported for cluster sizes larger than 4K, because

• Compression causes a performance degradation.

• The reason you use very large (>4K) clusters is to improve performance in some special cases.

Therefore, you should not use compression and large clusters at the same time. In other words, compression performs reasonably well on sizes up to 4K, but beyond that size, the savings in disk space is not worth the decrease in performance. When the cluster size is >4K on an NTFS volume, none of the NTFS compression functions are available.

For information about creating and formatting volumes, see "Creating and Formatting Volumes," in Chapter 17, "Disk and File System Basics."

Note

Windows NT does not support DoubleSpace or DriveSpace compression.