File Compression: What it is and why you probably need it -
By Janet Johnson, Now Software
Most Mac users come to a point in their lives where they realize they either: 1) need more room on their hard drives; or 2) need to speed up the way they work across networks or phone lines. Compression software can be the best way to maximize your disk space, as well as create smaller files to speed file transfer times. Just as you don’t have to be an engineer to need more room on your Mac, you don’t have to be an engineer to understand and use compression software. This is a quick primer on a little-understood but incredibly useful type of utility.
Simply, compression makes your files smaller. Smaller files take up less storage space, and move across phone lines and networks faster. So when you use compression software, you’ll maximize the space on your existing storage hardware, and save money on expensive connect time when you send files with your modem.
Compression works by looking at files and replacing repetitive bits of information with simple codes, thereby reducing the size of files while retaining their original integrity.
A simple example: The sentence ‘That’s one small step for man, and one giant leap for mankind’ could be compressed by replacing ‘one’ with ‘$’, ‘for man’ with ‘%’ and ‘nd’ with • to result in ‘That’s $ small step %, a• $ giant leap %ki•’.
The symbols $, •, and % take up only one bit of space each, but each symbol replaces a multi-letter word, so the resulting sentence is smaller. That, in a nutshell, is how compression works.
Types of Compression:
There are two types of compression for the Macintosh: driver-level and file-level compression. Driver-level compression has been around for some time for DOS machines, and it is now being introduced to the Macintosh market. Driver-level compression makes your operating system think your hard drive is twice as big as it actually is. Every file on your system is compressed, and your Mac must decompress and recompress each file as it is used.
File-level compression is the most common type of compression available for the Macintosh. File-level compressors compress and verify each file individually, helping to safeguard your data. File-level compression can occur in two ways: at idle-time or on-demand compression.
Idle-time compression works when your Mac is not being used, much like a screen saver does. You tell your software to compress files you haven’t used for a long time, or don’t use often, and the files are compressed in the background. This type of compression maximizes your hard disk space, and produces transparently-compressed files, which look just like regular files to your Mac. In other words, you can double-click on them from the Finder, or open them from within your applications, and they open just like regular files. After you close the files, they will be recompressed when your Mac is idle again. Because these files must be decompressed before they can be used, the speed of the decompression software is very important, so that using these files doesn’t interrupt your normal work flow.
On-demand compression, as the name implies, allows you to create compressed files on the spot, whenever you want. This technique is usually used to create the type of compressed files which speed up modem and network transfer times. These files, or collections of files, are typically called archives. The most important attribute of an archive is tightness. The smaller the archive, the faster it transmits. An archive can be self-extracting, which means that decompression instructions are included in the file, so the person receiving it can simply double-click on the archive and it will be decompressed without having the compression software installed.
Safety considerations
File integrity has sometimes been raised as a concern in the compression arena. The reason is that compression inherently reduces the redundancy of a file, and, as we’ve seen above, changes the context of its contents. Consequently, it is more difficult to reconstruct a compressed file that has been corrupted or damaged than an uncompressed file that has been corrupted. The key is to eliminate the chance of damage.
There are really three aspects to consider regarding the safety of compression software. The first concerns the level at which the compression takes place. For file-level compressors, each file is compressed individually. This approach is generally viewed as being safer than driver-level compression, since a single glitch at the driver level can affect all the data on an entire hard drive.
The second safety aspect concerns error-checking and data integrity. In the case of most file-level products, each file is checksummed and is then verified both before and after it is written to disk. To avoid data loss in the case of power outages, the original copy of a file being compressed should be preserved until compression is complete (note: not all file-level compressors work this way).
The third concern is user control. You need to have the final say over what gets compressed. In both idle-time and on-demand compression modes, you must decide whether to compress the whole disk, or to manually select specific files or folders to be compressed. You should be able to exclude individual files and folders from compression. For example, your current project might be a set of files you may not want to compress. This level of control provides you the comfort of knowing that files designated not to be compressed will be left alone. Keep in mind that even by selectively compressing files, it is not unusual to recover 30%-40% of your hard disk capacity without a hardware investment!
Why you need compression
As you add fonts, sounds, screen savers, images and applications to your Mac, your hard disk will fill up more quickly than you probably thought it could. Compression software can shrink the size of your files and applications, giving you more room on your drive. Modem and PowerBook users who travel and send files over modems can compress those files and save money on expensive long-distance rates. (An added bonus for PowerBook users who need to conserve battery power: Try storing transparently-compressed files in a RAM disk on your Powerbook, and see how much battery power you’ll save, while efficiently using your available RAM!) Network users will save time sending smaller files around the network, and because of that, network
performance is improved.
Selecting the compression product that’s right for you
Try to find one which does both idle-time and on-demand compression, because even if you don’t have a modem, and aren’t connected to a network now, archives are great for doing backups to floppies. Get a program which gives you lots of control over which files are compressed, and when files are compressed or not compressed. Look for a program which verifies each file and (in idle-time mode) works on copies of files rather than the originals — most file-level compression products provide that safety feature. And finally, how easy is the program to set up and use? Since compression is supposed to speed up the use of your Macintosh, make sure that the software you choose won’t bog you down with a hard-to-understand interface or confusing options.
Reclaim your hard disk!
Compression software is becoming increasingly popular on the Macintosh. Unless you have unlimited cash to spend on additional hard drives and CPU accelerators, a compression product is a wise investment for your system. So get a compression product and get all the storage space you paid for!
