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d963882053
#25147 added the translog deletion policy but didn't enable it by default. This PR enables a default retention of 512MB (same maximum size of the current translog) and an age of 12 hours (i.e., after 12 hours all translog files will be deleted). This increases to chance to have an ops based recovery, even if the primary flushed or the replica was offline for a few hours. In order to see which parts of the translog are committed into lucene the translog stats are extended to include information about uncommitted operations. Views now include all translog ops and guarantee, as before, that those will not go away. Snapshotting a view allows to filter out generations that are not relevant based on a specific sequence number. Relates to #10708
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,js]
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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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