Comment 2 for bug 1094271

It's funny I was playing today with autofs and had the same doubt about that folder and trying to clarify it I found your bug file.

I kept reading the documentation for a while and If I could understand it the auto.master.d directory is supposed to hold map files for autofs. This way, instead of editing auto.master file to include your mappings you can create an independent file inside auto.master.d. Then, when performing a system upgrade you won't risk to loose your manually included mappings, as auto.master remains unmodified.

For instance, I manually create the auto.master.d directory and a file inside named smb.autofs, with one single line:

/media/smb /etc/auto.smb --timeout=300,--ghost