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Translog
editTranslog
editChanges 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.
Flush settings
editThe following 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
.
Translog settings
editThe 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 index, delete,
update, or 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 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 to5s
. Values less than100ms
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 everysync_interval
. In the event of hardware failure, all acknowledged writes since the last automatic commit will be discarded.
-
What to do if the translog becomes corrupted?
editIn 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
.
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:
$ 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.