Understanding NTFS File Compression

by Joe Froehlich

 

I. Compression reduces the amount of disk space re­quired to store data. Under NTFS, compression is a file attribute that you can set on a per-file, per-directory, or per-disk basis. When you open a compressed file, it's automatically decompressed. And, when you close or save the file, it's automatically compressed again.

 

II. In this article, we'll present some guidelines for implementing NTFS compression on your workstations and servers. Then, we'll show you how to apply the compres­sion attribute to files, directories, and disks. Fi­nally, we'll discuss the effects of copying and moving compressed files between partitions.

 

III. There are a number of products on the market today that provide compression. Perhaps the most familiar of these is the technology found in PKZip and similar prod­ucts. One thing is true of any kind of compression — you always have to balance size and speed. In other words, if you want to maximize compression, the time it takes will increase. Conversely, if you want to maximize speed, the compression ratio will be smaller. The designers of NT opted for speed over reduced file size. You should be aware that there can be a significant performance penalty when you implement file compression. When NTFS reads a compressed file, it must decompress it before it presents it to the application. When NTFS writes a file, it must compress it again before it's written to disk. In gen­eral, servers that provide access to files that don't require frequent updates won't experience much of a per­formance bottleneck. However, the performance of servers that have a lot of read and write activity will degrade when implementing compression.

IV. Having said that, compression is fine for files that aren't accessed frequently, but you probably shouldn't apply compression to databases, shared docu­ments, or executables that run over the network. Also, you shouldn't compress the system partition's root di­rectory or paging files. Now, that we've examined some general guidelines for using compression, let's look at how to implement it.

V. To compress a file, right-click on the file and choose Properties. Next, select the Compressed check box. When you select a file to compress and click Apply, you'11 see the size of the compressed file next to the Compressed Size information item.

VI. To compress a directory, right-click on the direc-. tory and choose Properties. Next, select the Compress check box. When you select a directory to compress and click Apply, Windows NT displays a prompt asking if you want to compress all subdirectories within the directory.

VII. To compress a disk, right-click on the disk and choose Properties. Next, select the Compress check box. When you compress a disk, Windows NT displays a prompt asking if you want to compress all directories on the disk.

VIII. You can'configure Windows NT Explorer to display compressed files in an alternate color. When you do, uncompressed files and directories will still appear in black. The default color for compressed files is blue, so don't set the background color of your windows to blue.

Setting the display options:

IX. To set the display options, first launch NT Ex­plorer, and then select View|Options. Click the DisplayCompressed Files And Folders With Alternate Color checkbox, and click OK. Now, if you use NT Explorer's Detail view to list files, you'll see that the compression attribute (C) appears in the Attributes column.

X. It's important to remember that compression is an attribute. When you copy or move files and directories, the compression attribute is retained or lost in the same way as it is for other attributes. If you create a new file, the new file inherits the compression at­tribute of its target folder. If you move a file, the file retains its original compression attribute. Keep in mind also that, unlike the NTFS file system, FAT doesn't support compression. Table A summarizes the effect that various copy and move operations have on a compressed file and its compression attribute.