[sjwright]

Here's the essential business logic specification for HFS+ automatic de-fragmentation, for the curious.

It's actually quite a clever spec, because it takes advantage of existing efforts to read fragmented files to perform the majority of the de-fragmentation process.

I'm not sure if this spec applies to APFS or for SSDs. (With SSDs you're generally better off not defragmenting most of the time, because the performance penalty is far lower, but the write amplification has consequences.)

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When a file is opened on an HFS+ 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 3 mins

If all of the above conditions are satisfied, the file is relocated—it is defragmented on-the-fly.

http://osxbook.com/software/hfsdebug/fragmentation.html

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