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fcfd98e9e8
Today we have two variants of translogs for indexing. We only recommend the buffered one which also has a 20% advantage in indexing speed. This commit removes the option and defaults to the buffered case. It also hard-wires the translog buffer to 8kb instead of 64kb. We used to adjust that buffer based on if the shard is active or not, this code has also been removed and instead we just keep an 8kb buffer arround.
73 lines
2.8 KiB
Plaintext
73 lines
2.8 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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[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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committed 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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-- |