Translogedit

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.

Lucene commits are too expensive to perform on every individual change, so each shard copy also writes operations into its transaction log known as the translog. 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 operations that have been acknowledged but not yet included in the last Lucene commit are instead recovered from the translog when the shard recovers.

An Elasticsearch flush is the process of performing a Lucene commit and starting a new translog generation. Flushes are performed automatically in the background in order to make sure the translog does not 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.

Translog settingsedit

The data in the translog is only persisted to disk when the translog is fsynced and committed. In the event of a hardware failure or an operating system crash or a JVM crash or a shard 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 fsynced and committed on the primary and on every allocated replica. If index.translog.durability is set to async then Elasticsearch fsyncs and commits the translog only every index.translog.sync_interval which means that any operations that were performed just before a crash may be lost when the node recovers.

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

index.translog.sync_interval
How often the translog is fsynced 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 a 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 replayed if the shard was stopped and had to be recovered. This setting 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.

Translog retentionedit

deprecated::[7.4.0, translog retention settings are deprecated in favor of soft deletes. These settings are effectively ignored since 7.4 and will be removed in a future version].

If an index is not using soft deletes to retain historical operations then Elasticsearch recovers each replica shard by replaying operations from the primary’s translog. This means it is important for the primary to preserve extra operations in its translog in case it needs to rebuild a replica. Moreover it is important for each replica to preserve extra operations in its translog in case it is promoted to primary and then needs to rebuild its own replicas in turn. The following settings control how much translog is retained for peer recoveries.

index.translog.retention.size
This controls the total size of translog files to keep for each shard. Keeping more translog files increases the chance of performing an operation based sync when recovering a replica. If the translog files are not sufficient, replica recovery will fall back to a file based sync. Defaults to 512mb. This setting is ignored, and should not be set, if soft deletes are enabled. Soft deletes are enabled by default in indices created in Elasticsearch versions 7.0.0 and later.
index.translog.retention.age
This controls the maximum duration for which translog files are kept by each shard. 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 12h. This setting is ignored, and should not be set, if soft deletes are enabled. Soft deletes are enabled by default in indices created in Elasticsearch versions 7.0.0 and later.