title: What is happening when Spark is calling ShuffleBlockFetcherIterator?
body: <p>My spark job seems to spend alot of time getting blocks. Sometimes it will do this for an hour or 2. I have 1 partition for my dataset so I'm not sure why its doing so much shuffling. Anyone know what exactly is happening here?</p>
  
  <pre><code>15/12/16 18:05:27 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:05:27 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:05:27 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:05:40 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:05:40 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:05:40 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:05:40 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:05:59 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:05:59 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:05:59 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:05:59 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:06:13 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:06:13 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:06:13 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:06:13 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:06:33 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:06:33 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:06:33 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:06:33 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:06:49 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:06:49 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:06:49 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:06:49 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:07:14 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:07:14 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 1 ms
  15/12/16 18:07:14 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:07:14 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:07:33 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:07:33 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 1 ms
  15/12/16 18:07:33 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:07:33 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:07:46 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:07:46 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 1 ms
  15/12/16 18:07:47 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:07:47 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:07:58 INFO ShuffleBlockFetcherIterator: Getting 200 non-empty blocks out of 200 blocks
  15/12/16 18:07:58 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 0 ms
  15/12/16 18:07:58 INFO ShuffleBlockFetcherIterator: Getting 4 non-empty blocks out of 4 blocks
  15/12/16 18:07:58 INFO ShuffleBlockFetcherIterator: Started 0 remote fetches in 1 ms
  </code></pre>