anchor.diff:  GET /twitter/_stats/commit?level=shards
   --------------------------------------------------
   // AUTOSENSE
  +
  +
  +which returns something similar to:
  +
  +[source,js]
  +--------------------------------------------------
  +{
  +   ...
  +   "indices": {
  +      "twitter": {
  +         "primaries": {},
  +         "total": {},
  +         "shards": {
  +            "0": [
  +               {
  +                  "routing": {
  +                     ...
  +                  },
  +                  "commit": {
  +                     "id": "te7zF7C4UsirqvL6jp/vUg==",
  +                     "generation": 2,
  +                     "user_data": {
  +                        "sync_id": "AU2VU0meX-VX2aNbEUsD" <1>,
  +                        ...
  +                     },
  +                     "num_docs": 0
  +                  }
  +               }
  +               ...
  +            ],
  +            ...
  +         }
  +      }
  +   }
  +}
  +--------------------------------------------------
  +<1> the `sync id` marker
   
   [float]
   === Synced Flush API
   
   The Synced Flush API allows an administrator to initiate a synced flush manually. This can be particularly useful for
  -a planned (rolling) cluster restart where one can stop indexing and doesn't want to wait for the default 30 minutes to pass
  -when the synced flush will be performed automatically.
  +a planned (rolling) cluster restart where you can stop indexing and don't want to wait the default 30 minutes for
  +idle indices to be sync-flushed automatically.
   
   While handy, there are a couple of caveats for this API:
   
   1. Synced flush is a best effort operation. Any ongoing indexing operations will cause
  -the synced flush to fail. This means that some shards may be synced flushed while others aren't. See below for more.
  +the synced flush to fail on that shard. This means that some shards may be synced flushed while others aren't. See below for more.
   2. The `sync_id` marker is removed as soon as the shard is flushed again. That is because a flush replaces the low level
   lucene commit point where the marker is stored. Uncommitted operations in the transaction log do not remove the marker.
   In practice, one should consider any indexing operation on an index as removing the marker as a flush can be triggered by Elasticsearch
   at any time.
  +
  +
  +NOTE: It is harmless to request a synced flush while there is ongoing indexing. Shards that are idle will succeed and shards
  + that are not will fail. Any shards that succeeded will have faster recovery times.
   
   
   [source,bash]
  
positive.code: [[indices-synced-flush]]
  === Synced Flush
  
  Elasticsearch tracks the indexing activity of each shards. Shards that have not
  received any indexing operations for 30 minutes (configurable) are automatically marked as inactive. This presents
  an opportunity for Elasticsearch to reduce shard resources and also perform
  a special kind of flush, called `synced flush`. A synced flush performs normal
  flushing and adds a special uniquely generated marker (`sync_id`) to all shards.
  
  Since the sync id marker was added when there were no ongoing indexing operations, it can
  be used as a quick way to check if two shards indices are identical. This quick sync id
  comparison (if present) is used during recovery or restarts to skip the first and
  most costly phase of the process. In that case, no segment files need to be copied and
  the transaction log replay phase of the recovery can start immediately. Note that since the sync id
  marker was applied together with a flush, it is highly likely that the transaction log will be empty,
  speeding up recoveries even more.
  
  This is particularly useful for use cases having lots of indices which are
  never or very rarely updated, such as time based data. This use case typically generates lots of indices whose
  recovery without the synced flush marker would take a long time.
  
  To check whether a shard has a marker or not, one can use the `commit` section of shard stats returned by
  the <<indices-stats,indices stats>> API:
  
  [source,bash]
  
negative.code: --------------------------------------------------
  
  
  NOTE: When a shard copy fails to sync-flush, the HTTP status code returned will be `409 CONFLICT`.
  
  The synced flush API can be applied to more than one index with a single call,
  or even on `_all` the indices.
  
  