[[index-modules-translog]] == Translog Changes to Lucene are only persisted to disk during a Lucene commit, which is a relatively expensive operation and so cannot be performed after every index or delete operation. Changes that happen after one commit and before another will be removed from the index by Lucene in the event of process exit or hardware failure. Because Lucene commits are too expensive to perform on every individual change, each shard copy also has a _transaction log_ known as its _translog_ associated with it. All index and delete operations are written to the translog after being processed by the internal Lucene index but before they are acknowledged. In the event of a crash, recent transactions that have been acknowledged but not yet included in the last Lucene commit can instead be recovered from the translog when the shard recovers. An Elasticsearch flush is the process of performing a Lucene commit and starting a new translog. Flushes are performed automatically in the background in order to make sure the translog doesn't grow too large, which would make replaying its operations take a considerable amount of time during recovery. The ability to perform a flush manually is also exposed through an API, although this is rarely needed. [float] === Translog settings The data in the translog 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, `index.translog.durability` is set to `request` meaning that Elasticsearch will only report success of an index, delete, update, or bulk request to the client after the translog has been successfully ++fsync++ed and committed on the primary and on every allocated replica. If `index.translog.durability` is set to `async` then Elasticsearch ++fsync++s and commits the translog every `index.translog.sync_interval` (defaults to 5 seconds). The following <> per-index settings control the behaviour of the translog: `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. -- `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`:: The maximum duration for which translog files will be kept. Defaults to `12h`.