137 lines
6.2 KiB
Plaintext
137 lines
6.2 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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=== 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 if `index.translog.durability` is set
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to `async` or if set to `request` (default) 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`. Values less than `100ms` are not allowed.
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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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--
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`index.translog.flush_threshold_size`::
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The translog stores all operations that are not yet safely persisted in Lucene (i.e., are
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not part of a lucene commit point). Although these operations are available for reads, they will
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need to be reindexed if the shard was to shutdown and has to be recovered. This settings controls
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the maximum total size of these operations, to prevent recoveries from taking too long. Once the
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maximum size has been reached a flush will happen, generating a new Lucene commit. Defaults to `512mb`.
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`index.translog.retention.size`::
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The total size of translog files to keep. Keeping more translog files increases the chance of performing
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an operation based sync when recovering replicas. If the translog files are not sufficient, replica recovery
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will fall back to a file based sync. Defaults to `512mb`
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`index.translog.retention.age`::
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The maximum duration for which translog files will be kept. Defaults to `12h`.
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[float]
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[[corrupt-translog-truncation]]
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=== What to do if the translog becomes corrupted?
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In some cases (a bad drive, user error) the translog can become corrupted. When
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this corruption is detected by Elasticsearch due to mismatching checksums,
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Elasticsearch will fail the shard and refuse to allocate that copy of the data
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to the node, recovering from a replica if available.
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If there is no copy of the data from which Elasticsearch can recover
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successfully, a user may want to recover the data that is part of the shard at
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the cost of losing the data that is currently contained in the translog. We
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provide a command-line tool for this, `elasticsearch-translog`.
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[WARNING]
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The `elasticsearch-translog` tool should *not* be run while Elasticsearch is
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running, and you will permanently lose the documents that were contained only in
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the translog!
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In order to run the `elasticsearch-translog` tool, specify the `truncate`
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subcommand as well as the directory for the corrupted translog with the `-d`
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option:
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[source,txt]
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--------------------------------------------------
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$ bin/elasticsearch-translog truncate -d /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/
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Checking existing translog files
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!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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! WARNING: Elasticsearch MUST be stopped before running this tool !
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! !
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! WARNING: Documents inside of translog files will be lost !
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! !
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! WARNING: The following files will be DELETED! !
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!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-41.ckp
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--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-6.ckp
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--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-37.ckp
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--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-24.ckp
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--> data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-11.ckp
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Continue and DELETE files? [y/N] y
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Reading translog UUID information from Lucene commit from shard at [data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/index]
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Translog Generation: 3
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Translog UUID : AxqC4rocTC6e0fwsljAh-Q
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Removing existing translog files
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Creating new empty checkpoint at [data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog.ckp]
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Creating new empty translog at [data/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/translog-3.tlog]
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Done.
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--------------------------------------------------
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You can also use the `-h` option to get a list of all options and parameters
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that the `elasticsearch-translog` tool supports.
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