Does ext3 fragment when run at near-full capacity?

"Technically, yes, it can fragment if there is not much free space available, and no, it doesn't clean up after itself. To check the fragmentation level of a partition:

fsck -nvf /dev/sda1 # replace sda1 with the relevant partition

and to see how fragmented a particular file is:

filefrag -v /path/to/file

Here is an article about how to defrag a Linux system, but chances are it is not affecting your system to a noticeable degree, so you don't need to worry about it.

If you're really interested, this article and its follow up were extremely helpful to me in understanding how the file system works."