192 lines
6.3 KiB
Plaintext
192 lines
6.3 KiB
Plaintext
[[indices-flush]]
|
|
== Flush
|
|
|
|
The flush API allows to flush one or more indices through an API. The
|
|
flush process of an index basically frees memory from the index by
|
|
flushing data to the index storage and clearing the internal
|
|
<<index-modules-translog,transaction log>>. By
|
|
default, Elasticsearch uses memory heuristics in order to automatically
|
|
trigger flush operations as required in order to clear memory.
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
POST /twitter/_flush
|
|
--------------------------------------------------
|
|
// AUTOSENSE
|
|
|
|
[float]
|
|
[[flush-parameters]]
|
|
=== Request Parameters
|
|
|
|
The flush API accepts the following request parameters:
|
|
|
|
[horizontal]
|
|
`wait_if_ongoing`:: If set to `true` the flush operation will block until the
|
|
flush can be executed if another flush operation is already executing.
|
|
The default is `false` and will cause an exception to be thrown on
|
|
the shard level if another flush operation is already running.
|
|
|
|
`force`:: Whether a flush should be forced even if it is not necessarily needed ie.
|
|
if no changes will be committed to the index. This is useful if transaction log IDs
|
|
should be incremented even if no uncommitted changes are present.
|
|
(This setting can be considered as internal)
|
|
|
|
[float]
|
|
[[flush-multi-index]]
|
|
=== Multi Index
|
|
|
|
The flush API can be applied to more than one index with a single call,
|
|
or even on `_all` the indices.
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
POST /kimchy,elasticsearch/_flush
|
|
|
|
POST /_flush
|
|
--------------------------------------------------
|
|
// AUTOSENSE
|
|
|
|
[[indices-synced-flush]]
|
|
=== Synced Flush
|
|
|
|
Elasticsearch tracks the indexing activity of each shards. Shards that have not
|
|
received any indexing operations for 30 minutes (configurable) are automatically marked as inactive. This presents
|
|
an opportunity for Elasticsearch to reduce shard resources and also perform
|
|
a special kind of flush, called `synced flush`. A synced flush performs normal
|
|
flushing and adds a special uniquely generated marker (`sync_id`) to all shards.
|
|
|
|
Since the sync id marker was added when there were no ongoing indexing operations, it can
|
|
be used as a quick way to check if two shards indices are identical. This quick sync id
|
|
comparison (if present) is used during recovery or restarts to skip the first and
|
|
most costly phase of the process. In that case, no segment files need to be copied and
|
|
the transaction log replay phase of the recovery can start immediately. Note that since the sync id
|
|
marker was applied together with a flush, it is highly likely that the transaction log will be empty,
|
|
speeding up recoveries even more.
|
|
|
|
This is particularly useful for use cases having lots of indices which are
|
|
never or very rarely updated, such as time based data. This use case typically generates lots of indices whose
|
|
recovery without the synced flush marker would take a long time.
|
|
|
|
To check whether a shard has a marker or not, one can use the `commit` section of shard stats returned by
|
|
the <<indices-stats,indices stats>> API:
|
|
|
|
[source,bash]
|
|
--------------------------------------------------
|
|
GET /twitter/_stats/commit?level=shards
|
|
--------------------------------------------------
|
|
// AUTOSENSE
|
|
|
|
[float]
|
|
=== Synced Flush API
|
|
|
|
The Synced Flush API allows an administrator to initiate a synced flush manually. This can be particularly useful for
|
|
a planned (rolling) cluster restart where one can stop indexing and doesn't want to wait for the default 30 minutes to pass
|
|
when the synced flush will be performed automatically.
|
|
|
|
While handy, there are a couple of caveats for this API:
|
|
|
|
1. Synced flush is a best effort operation. Any ongoing indexing operations will cause
|
|
the synced flush to fail. This means that some shards may be synced flushed while others aren't. See below for more.
|
|
2. The `sync_id` marker is removed as soon as the shard is flushed again. That is because a flush replaces the low level
|
|
lucene commit point where the marker is stored. Uncommitted operations in the transaction log do not remove the marker.
|
|
In practice, one should consider any indexing operation on an index as removing the marker as a flush can be triggered by Elasticsearch
|
|
at any time.
|
|
|
|
|
|
[source,bash]
|
|
--------------------------------------------------
|
|
POST /twitter/_flush/synced
|
|
--------------------------------------------------
|
|
// AUTOSENSE
|
|
|
|
The response contains details about how many shards were successfully sync-flushed and information about any failure.
|
|
|
|
Here is what it looks like when all shards of a two shards and one replica index successfully
|
|
sync-flushed:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"_shards": {
|
|
"total": 4,
|
|
"successful": 4,
|
|
"failed": 0
|
|
},
|
|
"twitter": {
|
|
"total": 4,
|
|
"successful": 4,
|
|
"failed": 0
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
|
|
Here is what it looks like when one shard group failed due to pending operations:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"_shards": {
|
|
"total": 4,
|
|
"successful": 2,
|
|
"failed": 2
|
|
},
|
|
"twitter": {
|
|
"total": 4,
|
|
"successful": 2,
|
|
"failed": 2,
|
|
"failures": [
|
|
{
|
|
"shard": 1,
|
|
"reason": "[2] ongoing operations on primary"
|
|
}
|
|
]
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
|
|
Sometimes the failures are specific to a shard copy. The copies that failed will not be eligible for
|
|
fast recovery but those that succeeded still will be. This case is reported as follows:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"_shards": {
|
|
"total": 4,
|
|
"successful": 1,
|
|
"failed": 1
|
|
},
|
|
"twitter": {
|
|
"total": 4,
|
|
"successful": 3,
|
|
"failed": 1,
|
|
"failures": [
|
|
{
|
|
"shard": 1,
|
|
"reason": "unexpected error",
|
|
"routing": {
|
|
"state": "STARTED",
|
|
"primary": false,
|
|
"node": "SZNr2J_ORxKTLUCydGX4zA",
|
|
"relocating_node": null,
|
|
"shard": 1,
|
|
"index": "twitter"
|
|
}
|
|
}
|
|
]
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
|
|
The synced flush API can be applied to more than one index with a single call,
|
|
or even on `_all` the indices.
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
POST /kimchy,elasticsearch/_flush/synced
|
|
|
|
POST /_flush/synced
|
|
--------------------------------------------------
|
|
// AUTOSENSE |