I was running a compact command on a 2.4 collection imported into a 2.6.1 server.

The collection was ~ 56GiB and contained 31 datafiles originally.

When I ran compact, the command seems to go into a loop allocating new files datafiles:

2014-06-15T15:06:44.997-0400 [FileAllocator] allocating new datafile /data/mongo/dbpath/XXXXXXX.256, filling with zeroes...

I stopped the compact command at this point. This was also the second run of compact after resetting the db files (I stopped it at about 100 datafiles on that run).

Compacting the same collection on a 2.4.10 instance did not behave this way.