[[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] === Flush settings The following <> settings control how often the in-memory buffer is flushed to disk: `index.translog.flush_threshold_size`:: Once the translog hits this size, a flush will happen. Defaults to `512mb`. [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 <>, <>, <>, or <> 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 <> per-index settings control the behaviour of the transaction log: `index.translog.sync_interval`:: How often the translog is ++fsync++ed to disk and committed, regardless of write operations. Defaults to `5s`. Values less than `100ms` are not allowed. `index.translog.durability`:: + -- Whether or not to `fsync` and commit the translog after every index, delete, update, or bulk request. This setting accepts the following parameters: `request`:: (default) `fsync` and commit after every request. In the event of hardware failure, all acknowledged writes will already have been committed to disk. `async`:: `fsync` and commit in the background every `sync_interval`. In the event of hardware failure, all acknowledged writes since the last automatic commit will be discarded. --