99 lines
4.5 KiB
Plaintext
99 lines
4.5 KiB
Plaintext
[[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 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
|
|
++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 <<indices-update-settings,dynamically updatable>> 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 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`::
|
|
|
|
When soft deletes is disabled (enabled by default in 7.0 or later),
|
|
`index.translog.retention.size` controls 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`
|
|
|
|
Both `index.translog.retention.size` and `index.translog.retention.age` should not
|
|
be specified unless soft deletes is disabled as they will be ignored.
|
|
|
|
|
|
`index.translog.retention.age`::
|
|
|
|
When soft deletes is disabled (enabled by default in 7.0 or later),
|
|
`index.translog.retention.age` controls the maximum duration for which 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 `12h`
|
|
|
|
Both `index.translog.retention.size` and `index.translog.retention.age` should not
|
|
be specified unless soft deletes is disabled as they will be ignored.
|