On-the-fly Defragmentation
When a file is opened on an HFS Plus volume, the following conditions are tested
If the file is less than 20 MB in size
If the file is not already busy
If the file is not read-only
If the file has more than eight extents
If the system has been up for at least three minutes
I all of the above conditions are satisfied, the file is relocated -- it is defragmented on-the-fly.
File contiguity (regardless of file size) is promoted in general as a consequence of the extent-based allocation policy in HFS Plus, which also delays actua allocation