Windows 7 / Getting Started

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Zipping and Compressing

Windows 7 supports two very different kinds of file compression. The distinction is confusing but important.

File compression reduces the size of a file by cleverly taking out parts of the contents of the file that aren't needed, storing only the minimum amount of information necessary to reconstitute the file - extract it - into its full, original form. A certain amount of overhead is involved because the computer has to take the time to squeeze extraneous information out of a file before storing it, and then the computer takes more time to restore the file to its original state when someone needs the file. But compression can reduce file sizes enormously. A compressed file often takes up half its original space - even less, in many cases.

How does compression work? That depends on the compression method you use. In one kind of compression, known as Huffman encoding, letters that occur frequently in a file (say, the letter e in a word-processing document) are massaged so that they take up only a little bit of room in the file, whereas letters that occur less frequently (say, x) are allowed to occupy lots of space. Rather than allocate eight 1s and 0s for every letter in a document, for example, some letters may take up only two 1s and 0s and others could take up 15. The net result, overall, is a big reduction in file size. It's complicated, and the mathematics involved get quite interesting.

The two Windows 7 file compression techniques are as follows:

• Files can be compressed and placed in a Compressed (zipped) Folder, with an icon to match.
• Files, folders, or even entire drives can be compressed by using the built-in compression capabilities of the Windows 7 file system (NTFS).

Here's where things get complicated.

NT File System (NTFS) compression is built into the file system: You can use it only on NTFS drives, and the compression doesn't persist when you move (or copy) the file off the drive. Think of NTFS compression as a capability inherent to the hard drive itself. That isn't really the case - Windows 7 does all the sleight-of-hand behind the scenes - but the concept can help you remember the limitations and quirks of NTFS compression.

Although Microsoft would have you believe that Compressed (zipped) Folder compression is based on folders, it isn't. A Compressed (zipped) Folder is really a file - not a folder - but it's a special kind of file, called a Zip file. If you ever encountered Zip files on the Internet (they have a .zip filename extension and are frequently manipulated by using programs such as WinZip, www.winzip.com), you know exactly what I'm talking about. Zip files contain one or more compressed files, and they use the most common kind of compression found on the Internet. Think of Compressed (zipped) Folders as being Zip files, and if you have even a nodding acquaintance with Zips, you'll immediately understand the limitations and quirks of Compressed (zipped) Folders. Microsoft calls them Folders because that's supposed to be easier for users to understand. You be the judge.

Table below shows a quick comparison of NTFS compression and Zip compression.

NTFS Compression versus Compressed (Zipped) Folders Compression

NTFS	Zip
Think of NTFS compression as a feature of the hard drive itself.	Zip technology works on any file, regardless of where it is stored.
The minute you move an NTFScompressed file off an NTFS drive - by, say, sending a file as an e-mail attachment - the file is uncompressed, automatically, and you can't do anything about it: You'll send a big, uncompressed file.	You can move a Compressed (zipped) Folder (it's a Zip file, with a .zip filename extension) anywhere, and it stays compressed. If you send a Zip file as an e-mail attachment, it goes over the ether as a compressed file. The person who receives the file can view it directly in Windows 7, or he can use a product such as WinZip to see it.
A lot of overhead is associated with NTFS compression: Windows has to compress and decompress those files on the fly, and that sucks up processing power.	Very little overhead is associated with Zip files. Many programs (for example, antivirus programs) read Zip files directly.
NTFS compression is helpful if you're running out of room on an NTFSformatted drive.	Compressed (zipped) Folders (that is to say, Zip files) are in a near-universal form that can be used just about anywhere.
You have to be using an administrator account to use NTFS compression.	You can create, copy, or move Zip files just like any other files, with the same security restrictions.
You can use NTFS compression on entire drives, folders, or single files. They cannot be password protected.	You can zip files or folders, or (rarely) drives, and they can be password protected.

Warning If you try to compress the drive that contains your Windows folder, you can't compress the files that are in use by Windows.