This is possible if you run MR while inserting at a high rate

124600/1512823 8%
288000/1512823 19%
Wed Jul 28 13:07:56 [initandlisten] connection accepted from 127.0.0.1:43003 #3
476400/1512823 31%
657200/1512823 43%
827600/1512823 54%
1004100/1512823 66%
1182600/1512823 78%
1361200/1512823 89%
1529400/1512823 101%
1697500/1512823 112%
1865800/1512823 123%
2033300/1512823 134%
2198000/1512823 145%
2376200/1512823 157%
2557900/1512823 169%
2736000/1512823 180%
2899400/1512823 191%
3072800/1512823 203%
Wed Jul 28 13:08:46 allocating new datafile /data/db/test.4, filling with zeroes...
Wed Jul 28 13:08:46 done allocating datafile /data/db/test.4, size: 1024MB, took 0 secs
3247900/1512823 214%
3418600/1512823 225%
3600800/1512823 238%