110 lines
3.7 KiB
Plaintext
110 lines
3.7 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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`index.translog.flush_threshold_period`::
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How long to wait before triggering a flush regardless of translog size. Defaults to `30m`.
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`index.translog.interval`::
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How often to check if a flush is needed, randomized between the interval value
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and 2x the interval value. Defaults to `5s`.
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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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