100 lines
3.5 KiB
Plaintext
100 lines
3.5 KiB
Plaintext
[[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`.
|
|
|
|
`index.translog.flush_threshold_ops`::
|
|
|
|
After how many operations to flush. Defaults to `unlimited`.
|
|
|
|
[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
|
|
and 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`.
|
|
|
|
`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.
|
|
--
|
|
|
|
`index.translog.fs.type`::
|
|
+
|
|
--
|
|
|
|
Whether to buffer writes to the transaction log in memory or not. This
|
|
setting accepts the following parameters:
|
|
|
|
`buffered`::
|
|
|
|
(default) Translog writes first go to a 64kB buffer in memory,
|
|
and are only written to the disk when the buffer is full, or when an
|
|
`fsync` is triggered by a write request or the `sync_interval`.
|
|
|
|
`simple`::
|
|
|
|
Translog writes are written to the file system immediately, without
|
|
buffering. However, these writes will only be persisted to disk when an
|
|
`fsync` and commit is triggered by a write request or the `sync_interval`.
|
|
|
|
--
|