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.

Flush settingsedit

The following dynamically updatable settings control how often the in-memory buffer is flushed to disk:

Once the translog hits this size, a flush will happen. Defaults to 512mb.
After how many operations to flush. Defaults to unlimited.
How long to wait before triggering a flush regardless of translog size. Defaults to 30m.
How often to check if a flush is needed, randomized between the interval value and 2x the interval value. Defaults to 5s.

Translog settingsedit

The data in the transaction log is only persisted to disk when the translog is fsynced and committed. In the event of hardware failure, any data written since the previous translog commit will be lost.

By default, Elasticsearch fsyncs and commits the translog every 5 seconds and at the end of every index, delete, update, or 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 fsynced and committed on the primary and on every allocated replica.

The following dynamically updatable per-index settings control the behaviour of the transaction log:

How often the translog is fsynced to disk and committed, regardless of write operations. Defaults to 5s.

Whether or not to fsync and commit the translog after every index, delete, update, or bulk request. This setting accepts the following parameters:

(default) fsync and commit after every request. In the event of hardware failure, all acknowledged writes will already have been committed to disk.
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.

Whether to buffer writes to the transaction log in memory or not. This setting accepts the following parameters:

(default) Translog writes first go to a 64kB buffer in memory, and are only written to the disk when the buffer is full, or when an fsync is triggered by a write request or the sync_interval.
Translog writes are written to the file system immediately, without buffering. However, these writes will only be persisted to disk when an fsync and commit is triggered by a write request or the sync_interval.