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39e81c3ca6
This commit updates the docs about translog retention and flushing to reflect recent changes in how peer recoveries work. It also adds some docs to describe how history is retained for replay using soft deletes and shard history retention leases. Relates #45473
266 lines
9.3 KiB
Plaintext
266 lines
9.3 KiB
Plaintext
[[indices-flush]]
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=== Flush
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Flushing an index is the process of making sure that any data that is currently
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only stored in the <<index-modules-translog,transaction log>> is also
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permanently stored in the Lucene index. When restarting, {es} replays any
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unflushed operations from the transaction log into the Lucene index to bring it
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back into the state that it was in before the restart. {es} automatically
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triggers flushes as needed, using heuristics that trade off the size of the
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unflushed transaction log against the cost of performing each flush.
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Once each operation has been flushed it is permanently stored in the Lucene
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index. This may mean that there is no need to maintain an additional copy of it
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in the transaction log, unless <<index-modules-translog-retention,it is retained
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for some other reason>>. The transaction log is made up of multiple files,
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called _generations_, and {es} will delete any generation files once they are no
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longer needed, freeing up disk space.
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It is also possible to trigger a flush on one or more indices using the flush
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API, although it is rare for users to need to call this API directly. If you
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call the flush API after indexing some documents then a successful response
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indicates that {es} has flushed all the documents that were indexed before the
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flush API was called.
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[source,js]
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--------------------------------------------------
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POST twitter/_flush
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--------------------------------------------------
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// CONSOLE
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// TEST[setup:twitter]
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[float]
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[[flush-parameters]]
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==== Request Parameters
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The flush API accepts the following request parameters:
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[horizontal]
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`wait_if_ongoing`:: If set to `true` the flush operation will block until the
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flush can be executed if another flush operation is already executing. If set to
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`false` then an exception will be thrown on the shard level if another flush
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operation is already running. Defaults to `true`.
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`force`:: Whether a flush should be forced even if it is not necessarily needed
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i.e. if no changes will be committed to the index. This can be used to force
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the generation number of the transaction log to be incremented even if no
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uncommitted changes are present. This parameter should be considered internal.
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[float]
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[[flush-multi-index]]
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==== Multi Index
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The flush API can be applied to more than one index with a single call, or even
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on `_all` the indices.
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[source,js]
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--------------------------------------------------
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POST kimchy,elasticsearch/_flush
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POST _flush
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--------------------------------------------------
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// CONSOLE
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// TEST[s/^/PUT kimchy\nPUT elasticsearch\n/]
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[[synced-flush-api]]
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==== Synced Flush
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{es} keeps track of which shards have received indexing activity recently, and
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considers shards that have not received any indexing operations for 5 minutes to
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be inactive. When a shard becomes inactive {es} performs a special kind of flush
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known as a _synced flush_. A synced flush performs a normal
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<<indices-flush,flush>> on each copy of the shard, and then adds a marker known
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as the `sync_id` to each copy to indicate that these copies have identical
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Lucene indices. Comparing the `sync_id` markers of the two copies is a very
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efficient way to check whether they have identical contents.
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When allocating shard copies, {es} must ensure that each replica contains the
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same data as the primary. If the shard copies have been synced-flushed and the
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replica shares a `sync_id` with the primary then {es} knows that the two copies
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have identical contents. This means there is no need to copy any segment files
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from the primary to the replica, which saves a good deal of time during
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recoveries and restarts.
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This is particularly useful for clusters having lots of indices which are very
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rarely updated, such as with time-based indices. Without the synced flush
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marker, recovery of this kind of cluster would be much slower.
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To check whether a shard has a `sync_id` marker or not, look for the `commit`
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section of the shard stats returned by the <<indices-stats,indices stats>> API:
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[source,sh]
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--------------------------------------------------
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GET twitter/_stats?filter_path=**.commit&level=shards <1>
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--------------------------------------------------
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// CONSOLE
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// TEST[s/^/PUT twitter\nPOST twitter\/_flush\/synced\n/]
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<1> `filter_path` is used to reduce the verbosity of the response, but is entirely optional
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which returns something similar to:
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[source,js]
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--------------------------------------------------
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{
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"indices": {
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"twitter": {
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"shards": {
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"0": [
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{
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"commit" : {
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"id" : "3M3zkw2GHMo2Y4h4/KFKCg==",
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"generation" : 3,
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"user_data" : {
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"translog_uuid" : "hnOG3xFcTDeoI_kvvvOdNA",
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"history_uuid" : "XP7KDJGiS1a2fHYiFL5TXQ",
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"local_checkpoint" : "-1",
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"translog_generation" : "2",
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"max_seq_no" : "-1",
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"sync_id" : "AVvFY-071siAOuFGEO9P", <1>
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"max_unsafe_auto_id_timestamp" : "-1",
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"min_retained_seq_no" : "0"
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},
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"num_docs" : 0
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}
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}
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]
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}
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}
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}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"id" : "3M3zkw2GHMo2Y4h4\/KFKCg=="/"id": $body.indices.twitter.shards.0.0.commit.id/]
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// TESTRESPONSE[s/"translog_uuid" : "hnOG3xFcTDeoI_kvvvOdNA"/"translog_uuid": $body.indices.twitter.shards.0.0.commit.user_data.translog_uuid/]
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// TESTRESPONSE[s/"history_uuid" : "XP7KDJGiS1a2fHYiFL5TXQ"/"history_uuid": $body.indices.twitter.shards.0.0.commit.user_data.history_uuid/]
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// TESTRESPONSE[s/"sync_id" : "AVvFY-071siAOuFGEO9P"/"sync_id": $body.indices.twitter.shards.0.0.commit.user_data.sync_id/]
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<1> the `sync id` marker
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NOTE: The `sync_id` marker is removed as soon as the shard is flushed again, and
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{es} may trigger an automatic flush of a shard at any time if there are
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unflushed operations in the shard's translog. In practice this means that one
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should consider any indexing operation on an index as having removed its
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`sync_id` markers.
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[float]
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==== Synced Flush API
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The Synced Flush API allows an administrator to initiate a synced flush
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manually. This can be particularly useful for a planned cluster restart where
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you can stop indexing but don't want to wait for 5 minutes until all indices
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are marked as inactive and automatically sync-flushed.
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You can request a synced flush even if there is ongoing indexing activity, and
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{es} will perform the synced flush on a "best-effort" basis: shards that do not
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have any ongoing indexing activity will be successfully sync-flushed, and other
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shards will fail to sync-flush. The successfully sync-flushed shards will have
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faster recovery times as long as the `sync_id` marker is not removed by a
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subsequent flush.
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[source,sh]
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--------------------------------------------------
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POST twitter/_flush/synced
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--------------------------------------------------
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// CONSOLE
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// TEST[setup:twitter]
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The response contains details about how many shards were successfully
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sync-flushed and information about any failure.
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Here is what it looks like when all shards of a two shards and one replica
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index successfully sync-flushed:
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[source,js]
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--------------------------------------------------
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{
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"_shards": {
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"total": 2,
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"successful": 2,
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"failed": 0
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},
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"twitter": {
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"total": 2,
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"successful": 2,
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"failed": 0
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}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"successful": 2/"successful": 1/]
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Here is what it looks like when one shard group failed due to pending
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operations:
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[source,js]
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--------------------------------------------------
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{
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"_shards": {
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"total": 4,
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"successful": 2,
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"failed": 2
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},
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"twitter": {
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"total": 4,
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"successful": 2,
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"failed": 2,
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"failures": [
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{
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"shard": 1,
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"reason": "[2] ongoing operations on primary"
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}
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]
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}
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}
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--------------------------------------------------
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// NOTCONSOLE
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NOTE: The above error is shown when the synced flush fails due to concurrent
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indexing operations. The HTTP status code in that case will be `409 Conflict`.
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Sometimes the failures are specific to a shard copy. The copies that failed
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will not be eligible for fast recovery but those that succeeded still will be.
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This case is reported as follows:
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[source,js]
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--------------------------------------------------
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{
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"_shards": {
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"total": 4,
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"successful": 1,
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"failed": 1
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},
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"twitter": {
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"total": 4,
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"successful": 3,
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"failed": 1,
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"failures": [
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{
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"shard": 1,
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"reason": "unexpected error",
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"routing": {
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"state": "STARTED",
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"primary": false,
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"node": "SZNr2J_ORxKTLUCydGX4zA",
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"relocating_node": null,
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"shard": 1,
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"index": "twitter"
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}
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}
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]
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}
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}
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--------------------------------------------------
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// NOTCONSOLE
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NOTE: When a shard copy fails to sync-flush, the HTTP status code returned will
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be `409 Conflict`.
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The synced flush API can be applied to more than one index with a single call,
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or even on `_all` the indices.
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[source,js]
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--------------------------------------------------
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POST kimchy,elasticsearch/_flush/synced
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POST _flush/synced
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--------------------------------------------------
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// CONSOLE
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