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.
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, 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 index, delete,
update, or bulk request. More precisely, if set
to request
, 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.
The following dynamically updatable per-index settings control the behaviour of the translog:
index.translog.sync_interval
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
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
512mb
.
index.translog.retention.size
512mb
index.translog.retention.age
12h
.