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75e816400c
This commit moves the size and ops based flush into a synchronous API into IndexShard and removes the time-based flush alltogether since it' basically covered by the inactive async flush API we have today. The functionality doesn't need to be covered by scheduled task and async APIs while we can actually make all the decisions in a sync manner which is way easier to control and to test. Closes #13707
100 lines
3.5 KiB
Plaintext
100 lines
3.5 KiB
Plaintext
[[index-modules-translog]]
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== Translog
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Changes to Lucene are only persisted to disk during a Lucene commit,
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which is a relatively heavy operation and so cannot be performed after every
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index or delete operation. Changes that happen after one commit and before another
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will be lost in the event of process exit or HW failure.
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To prevent this data loss, each shard has a _transaction log_ or write ahead
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log associated with it. Any index or delete operation is written to the
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translog after being processed by the internal Lucene index.
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In the event of a crash, recent transactions can be replayed from the
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transaction log when the shard recovers.
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An Elasticsearch flush is the process of performing a Lucene commit and
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starting a new translog. It is done automatically in the background in order
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to make sure the transaction log doesn't grow too large, which would make
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replaying its operations take a considerable amount of time during recovery.
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It is also exposed through an API, though its rarely needed to be performed
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manually.
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[float]
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=== Flush settings
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The following <<indices-update-settings,dynamically updatable>> settings
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control how often the in-memory buffer is flushed to disk:
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`index.translog.flush_threshold_size`::
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Once the translog hits this size, a flush will happen. Defaults to `512mb`.
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`index.translog.flush_threshold_ops`::
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After how many operations to flush. Defaults to `unlimited`.
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[float]
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=== Translog settings
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The data in the transaction log is only persisted to disk when the translog is
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++fsync++ed and committed. In the event of hardware failure, any data written
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since the previous translog commit will be lost.
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By default, Elasticsearch ++fsync++s and commits the translog every 5 seconds
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and at the end of every <<docs-index_,index>>, <<docs-delete,delete>>,
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<<docs-update,update>>, or <<docs-bulk,bulk>> request. In fact, Elasticsearch
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will only report success of an index, delete, update, or bulk request to the
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client after the transaction log has been successfully ++fsync++ed and committed
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on the primary and on every allocated replica.
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The following <<indices-update-settings,dynamically updatable>> per-index settings
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control the behaviour of the transaction log:
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`index.translog.sync_interval`::
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How often the translog is ++fsync++ed to disk and committed, regardless of
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write operations. Defaults to `5s`.
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`index.translog.durability`::
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+
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--
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Whether or not to `fsync` and commit the translog after every index, delete,
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update, or bulk request. This setting accepts the following parameters:
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`request`::
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(default) `fsync` and commit after every request. In the event
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of hardware failure, all acknowledged writes will already have been
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commited to disk.
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`async`::
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`fsync` and commit in the background every `sync_interval`. In
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the event of hardware failure, all acknowledged writes since the last
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automatic commit will be discarded.
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--
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`index.translog.fs.type`::
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+
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--
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Whether to buffer writes to the transaction log in memory or not. This
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setting accepts the following parameters:
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`buffered`::
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(default) Translog writes first go to a 64kB buffer in memory,
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and are only written to the disk when the buffer is full, or when an
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`fsync` is triggered by a write request or the `sync_interval`.
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`simple`::
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Translog writes are written to the file system immediately, without
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buffering. However, these writes will only be persisted to disk when an
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`fsync` and commit is triggered by a write request or the `sync_interval`.
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--
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