[[index-modules-translog]]
== Translog

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.

[float]
=== Flush settings

The following <<indices-update-settings,dynamically updatable>> settings
control how often the in-memory buffer is flushed to disk:

`index.translog.flush_threshold_size`::

Once the translog hits this size, a flush will happen. Defaults to `512mb`.

[float]
=== Translog settings

The data in the transaction log 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 <<docs-index_,index>>, <<docs-delete,delete>>,
<<docs-update,update>>, or  <<docs-bulk,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 ++fsync++ed and committed
on the primary and on every allocated replica.

The following <<indices-update-settings,dynamically updatable>> per-index settings
control the behaviour of the transaction log:

`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 hardware failure, all acknowledged writes since the last
    automatic commit will be discarded.
--

[float]
[[corrupt-translog-truncation]]
=== What to do if the translog becomes corrupted?

In some cases (a bad drive, user error) the translog can become corrupted. When
this corruption is detected by Elasticsearch due to mismatching checksums,
Elasticsearch will fail the shard and refuse to allocate that copy of the data
to the node, recovering from a replica if available.

If there is no copy of the data from which Elasticsearch can recover
successfully, a user may want to recover the data that is part of the shard at
the cost of losing the data that is currently contained in the translog. We
provide a command-line tool for this, `elasticsearch-translog`.

[WARNING]
The `elasticsearch-translog` tool should *not* be run while Elasticsearch is
running, and you will permanently lose the documents that were contained only in
the translog!

In order to run the `elasticsearch-translog` tool, specify the `truncate`
subcommand as well as the directory for the corrupted translog with the `-d`
option:

[source,js]
--------------------------------------------------
$ bin/elasticsearch-translog truncate -d /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/
Checking existing translog files
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!   WARNING: Elasticsearch MUST be stopped before running this tool   !
!                                                                     !
!   WARNING:    Documents inside of translog files will be lost       !
!                                                                     !
!   WARNING:          The following files will be DELETED!            !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-41.ckp
--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-6.ckp
--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-37.ckp
--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-24.ckp
--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-11.ckp

Continue and DELETE files? [y/N] y
Reading translog UUID information from Lucene commit from shard at [data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/index]
Translog Generation: 3
Translog UUID      : AxqC4rocTC6e0fwsljAh-Q
Removing existing translog files
Creating new empty checkpoint at [data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog.ckp]
Creating new empty translog at [data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-3.tlog]
Done.
--------------------------------------------------

You can also use the `-h` option to get a list of all options and parameters
that the `elasticsearch-translog` tool supports.
