How much drive space is enough?

Question

A recent, now closed topic at http://discussions.apple.com/thread.jspa?threadID=434397 raises questions about OS X drive space use & how much should remain free for proper operation of the OS.
Many seem to have some opinion on this, but nobody seems to agree on the appropriate rule of thumb. Apple doesn't seem to offer much in the way of user friendly official guidelines, so I'm hoping this topic will inspire a more comprehensive discussion of the issues involved, or at least where to look for better answers.

Answer 1

To sort of add to Gulliver's information some related ideas:
1. OS X adds VM typically in 64 MB chunks if you're using Tiger. Panther and earlier versions added VM in chunks of 128 MBs. That means that as more VM is required you need sufficient disk space to eventually accomodate at least twice the amount of installed RAM in the most extreme situation. Hardly ever going to happen, but at least it represents a practical limit.
2. Software installers require from 2-3 times as much disk space during installation as the size of the installed item.
3. Temporary files, logs, cache files, etc. require varying amounts of disk space both for the active files as well as for archives. Yesterday, for example, I cleaned off all my user and font caches - nearly 100 MBs.
4. Applications create preference files and/or store data files in the Applications Support folders.
5. If you allow your free space to drop under 1 GB there is a growing risk of permanent disk corruption that will be caused if the operating system requires additional directory extents. Directory extents are allocated in 4 MB chunks and the chunk must be contiguous. If the OS is unable to allocate a contiguous 4 MB chunk for the disk directory, then the entire directory will become corrupted potentially causing a catastrophic loss of data. Despite self-defragmentation, some disk fragmentation will always exist, and as the free space on the hard drive declines so declines the liklihood of finding a contiguous chunk large enough to accomodate the directory expansion if it's needed.
All of this leads to a conclusion that there is no specific amount of free space you should maintain, but there are some practical constraints that are dependent partly on what you as a user do. To accomodate a wide range of users I recommend a minimum of 5-6 GBs or 15 percent of the hard drive's capacity whichever is larger.