anchor.diff:  [[corrupt-translog-truncation]]
   === What to do if the translog becomes corrupted?
   
  -In some cases (a bad drive, user error) the translog can become corrupted. When
  -this corruption is detected by Elasticsearch due to mismatching checksums,
  -Elasticsearch will fail the shard and refuse to allocate that copy of the data
  -to the node, recovering from a replica if available.
  +In some cases (a bad drive, user error) the translog on a shard copy can become
  +corrupted. When this corruption is detected by Elasticsearch due to mismatching
  +checksums, Elasticsearch will fail that shard copy and refuse to use that copy
  +of the data.  If there are other copies of the shard available then
  +Elasticsearch will automatically recover from one of them using the normal
  +shard allocation and recovery mechanism.  In particular, if the corrupt shard
  +copy was the primary when the corruption was detected then one of its replicas
  +will be promoted in its place.
   
   If there is no copy of the data from which Elasticsearch can recover
   successfully, a user may want to recover the data that is part of the shard at
  
positive.code: [[index-modules-translog]]
  == Translog
  
  Changes to Lucene are only persisted to disk during a Lucene commit,
  which is a relatively heavy operation and so cannot be performed after every
  index or delete operation. Changes that happen after one commit and before another
  will be lost in the event of process exit or HW failure.
  
  To prevent this data loss, each shard has a _transaction log_ or write ahead
  log associated with it. Any index or delete operation is written to the
  translog after being processed by the internal Lucene index.
  
  In the event of a crash, recent transactions can be replayed from the
  transaction log when the shard recovers.
  
  An Elasticsearch flush is the process of performing a Lucene commit and
  starting a new translog. It is done automatically in the background in order
  to make sure the transaction log doesn't grow too large, which would make
  replaying its operations take a considerable amount of time during recovery.
  It is also exposed through an API, though its rarely needed to be performed
  manually.
  
  [float]
  === Translog settings
  
  The data in the transaction log is only persisted to disk when the translog is
  ++fsync++ed and committed.  In the event of hardware failure, any data written
  since the previous translog commit will be lost.
  
  By default, Elasticsearch ++fsync++s and commits the translog every 5 seconds if `index.translog.durability` is set
  to `async` or if set to `request` (default) at the end of every <<docs-index_,index>>, <<docs-delete,delete>>,
  <<docs-update,update>>, or  <<docs-bulk,bulk>> request.  In fact, Elasticsearch
  will only report success of an index, delete, update, or bulk request to the
  client after the transaction log has been successfully ++fsync++ed and committed
  on the primary and on every allocated replica.
  
  The following <<indices-update-settings,dynamically updatable>> per-index settings
  control the behaviour of the transaction log:
  
  `index.translog.sync_interval`::
  
  
negative.code: 
  `index.translog.flush_threshold_size`::
  
  The translog stores all operations that are not yet safely persisted in Lucene
  (i.e., are not part of a Lucene commit point). Although these operations are
  available for reads, they will need to be reindexed if the shard was to
  shutdown and has to be recovered. This settings controls the maximum total size
  of these operations, to prevent recoveries from taking too long. Once the
  maximum size has been reached a flush will happen, generating a new Lucene
  commit point. Defaults to `512mb`.
  
  `index.translog.retention.size`::
  
  The total size of translog files to keep. Keeping more translog files increases
  the chance of performing an operation based sync when recovering replicas. If
  the translog files are not sufficient, replica recovery will fall back to a
  file based sync. Defaults to `512mb`
  
  
  `index.translog.retention.age`::
  