Translog
editTranslog
editChanges 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.
Translog settings
editThe 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 every index.translog.sync_interval (defaults to 5 seconds).
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 to5s. Values less than100msare not allowed. -
index.translog.durability -
Whether or not to
fsyncand commit the translog after every index, delete, update, or bulk request. This setting accepts the following parameters:-
request -
(default)
fsyncand commit after every request. In the event of hardware failure, all acknowledged writes will already have been committed to disk. -
async -
fsyncand commit in the background everysync_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 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.