anchor.diff:  * Each sub-request gets a slightly different snapshot of the source index,
   though these are all taken at approximately the same time.
   
  -[float]
   [[docs-reindex-picking-slices]]
   ====== Picking the number of slices
   
  
positive.code: The previous method can also be used in conjunction with <<docs-reindex-change-name, changing a field name>>
  to load only the existing data into the new index and rename any fields if needed.
  
  [float]
  ==== Extracting a random subset of an index
  
  `_reindex` can be used to extract a random subset of an index for testing:
  
  
negative.code: wait_time = target_time - write_time = 2 seconds - .5 seconds = 1.5 seconds
  --------------------------------------------------
  
  Since the batch is issued as a single `_bulk` request, large batch sizes 
  cause Elasticsearch to create many requests and then wait for a while before
  starting the next set. This is "bursty" instead of "smooth".
  
  [[docs-reindex-rethrottle]]
  ===== Rethrottling
  
  The value of `requests_per_second` can be changed on a running reindex using
  the `_rethrottle` API:
  