123 lines
5.2 KiB
Plaintext
123 lines
5.2 KiB
Plaintext
[[indices-shadow-replicas]]
|
|
== Shadow replica indices
|
|
|
|
deprecated[5.2.0, Shadow replicas don't see much usage and we are planning to remove them]
|
|
|
|
If you would like to use a shared filesystem, you can use the shadow replicas
|
|
settings to choose where on disk the data for an index should be kept, as well
|
|
as how Elasticsearch should replay operations on all the replica shards of an
|
|
index.
|
|
|
|
In order to fully utilize the `index.data_path` and `index.shadow_replicas`
|
|
settings, you need to allow Elasticsearch to use the same data directory for
|
|
multiple instances by setting `node.add_lock_id_to_custom_path` to false in
|
|
elasticsearch.yml:
|
|
|
|
[source,yaml]
|
|
--------------------------------------------------
|
|
node.add_lock_id_to_custom_path: false
|
|
--------------------------------------------------
|
|
|
|
You will also need to indicate to the security manager where the custom indices
|
|
will be, so that the correct permissions can be applied. You can do this by
|
|
setting the `path.shared_data` setting in elasticsearch.yml:
|
|
|
|
[source,yaml]
|
|
--------------------------------------------------
|
|
path.shared_data: /opt/data
|
|
--------------------------------------------------
|
|
|
|
This means that Elasticsearch can read and write to files in any subdirectory of
|
|
the `path.shared_data` setting.
|
|
|
|
You can then create an index with a custom data path, where each node will use
|
|
this path for the data:
|
|
|
|
[WARNING]
|
|
========================
|
|
Because shadow replicas do not index the document on replica shards, it's
|
|
possible for the replica's known mapping to be behind the index's known mapping
|
|
if the latest cluster state has not yet been processed on the node containing
|
|
the replica. Because of this, it is highly recommended to use pre-defined
|
|
mappings when using shadow replicas.
|
|
========================
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
curl -XPUT 'localhost:9200/my_index' -d '
|
|
{
|
|
"index" : {
|
|
"number_of_shards" : 1,
|
|
"number_of_replicas" : 4,
|
|
"data_path": "/opt/data/my_index",
|
|
"shadow_replicas": true
|
|
}
|
|
}'
|
|
--------------------------------------------------
|
|
|
|
[WARNING]
|
|
========================
|
|
In the above example, the "/opt/data/my_index" path is a shared filesystem that
|
|
must be available on every node in the Elasticsearch cluster. You must also
|
|
ensure that the Elasticsearch process has the correct permissions to read from
|
|
and write to the directory used in the `index.data_path` setting.
|
|
========================
|
|
|
|
The `data_path` does not have to contain the index name, in this case,
|
|
"my_index" was used but it could easily also have been "/opt/data/"
|
|
|
|
An index that has been created with the `index.shadow_replicas` setting set to
|
|
"true" will not replicate document operations to any of the replica shards,
|
|
instead, it will only continually refresh. Once segments are available on the
|
|
filesystem where the shadow replica resides (after an Elasticsearch "flush"), a
|
|
regular refresh (governed by the `index.refresh_interval`) can be used to make
|
|
the new data searchable.
|
|
|
|
NOTE: Since documents are only indexed on the primary shard, realtime GET
|
|
requests could fail to return a document if executed on the replica shard,
|
|
therefore, GET API requests automatically have the `?preference=_primary` flag
|
|
set if there is no preference flag already set.
|
|
|
|
In order to ensure the data is being synchronized in a fast enough manner, you
|
|
may need to tune the flush threshold for the index to a desired number. A flush
|
|
is needed to fsync segment files to disk, so they will be visible to all other
|
|
replica nodes. Users should test what flush threshold levels they are
|
|
comfortable with, as increased flushing can impact indexing performance.
|
|
|
|
The Elasticsearch cluster will still detect the loss of a primary shard, and
|
|
transform the replica into a primary in this situation. This transformation will
|
|
take slightly longer, since no `IndexWriter` is maintained for each shadow
|
|
replica.
|
|
|
|
Below is the list of settings that can be changed using the update
|
|
settings API:
|
|
|
|
`index.data_path` (string)::
|
|
Path to use for the index's data. Note that by default Elasticsearch will
|
|
append the node ordinal by default to the path to ensure multiple instances
|
|
of Elasticsearch on the same machine do not share a data directory.
|
|
|
|
`index.shadow_replicas`::
|
|
Boolean value indicating this index should use shadow replicas. Defaults to
|
|
`false`.
|
|
|
|
`index.shared_filesystem`::
|
|
Boolean value indicating this index uses a shared filesystem. Defaults to
|
|
the `true` if `index.shadow_replicas` is set to true, `false` otherwise.
|
|
|
|
`index.shared_filesystem.recover_on_any_node`::
|
|
Boolean value indicating whether the primary shards for the index should be
|
|
allowed to recover on any node in the cluster. If a node holding a copy of
|
|
the shard is found, recovery prefers that node. Defaults to `false`.
|
|
|
|
=== Node level settings related to shadow replicas
|
|
|
|
These are non-dynamic settings that need to be configured in `elasticsearch.yml`
|
|
|
|
`node.add_lock_id_to_custom_path`::
|
|
Boolean setting indicating whether Elasticsearch should append the node's
|
|
ordinal to the custom data path. For example, if this is enabled and a path
|
|
of "/tmp/foo" is used, the first locally-running node will use "/tmp/foo/0",
|
|
the second will use "/tmp/foo/1", the third "/tmp/foo/2", etc. Defaults to
|
|
`true`.
|