anchor.diff:  the whole data centre loses power at the same time, then it is possible that
   you could lose the last 5 seconds (or `sync_interval`) of data.
   
  -******************************************************
  +We are constantly monitoring the perfromance implications of better default
  +translog sync semantics, so the default might change as time passes and HW,
  +virtualization, and other aspects improve.
  +
  +******************************************************
positive.code: [[index-modules-translog]]
  == Translog
  
  Changes to a shard are only persisted to disk when the shard is ``flushed'',
  which is a relatively heavy operation and so cannot be performed after every
  index or delete operation. Instead, changes are accumulated in an in-memory
  indexing buffer and only written to disk periodically. This would mean that
  the contents of the in-memory buffer would be lost in the event of power
  failure or some other hardware crash.
  
  To prevent this data loss, each shard has a _transaction log_ or write ahead
  log associated with it. Any index or delete operation is first written to the
  translog before being processed by the internal Lucene index. This translog is
  only cleared once the shard has been flushed and the data in the in-memory
  buffer persisted to disk as a Lucene segment.
  
  In the event of a crash, recent transactions can be replayed from the
  transaction log when the shard recovers.
  
  [float]
  === Flush settings
  
negative.code: 
  `index.translog.sync_interval`::
  
  How often the translog is ++fsync++ed to disk. Defaults to `5s`. Can be set to
  `0` to sync after each operation.
  
  `index.translog.fs.type`::
  
  