Merge branch '345278-add-guide-to-docs-for-zerodowntime-reindexing' into 'master'

Add guide for using zero downtime reindexing

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/115351



Merged-by: Ashraf Khamis <akhamis@gitlab.com>
Approved-by: Dmitry Gruzd <dgruzd@gitlab.com>
Approved-by: Ashraf Khamis <akhamis@gitlab.com>
Reviewed-by: Terri Chu <tchu@gitlab.com>
Co-authored-by: Terri Chu <tchu@gitlab.com>

doc/integration/advanced_search/elasticsearch.md

@@ -418,7 +418,7 @@ To disable the Elasticsearch integration:
 1. Expand **Advanced Search**.
 1. Clear the **Pause Elasticsearch indexing** checkbox.
 
-## Zero downtime reindexing
+## Zero-downtime reindexing
 
 The idea behind this reindexing method is to leverage the [Elasticsearch reindex API](https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html)
 and Elasticsearch index alias feature to perform the operation. We set up an index alias which connects to a
@@ -427,6 +427,10 @@ the writes to the `primary` index. Then, we create another index and invoke the
 index data onto the new index. After the reindexing job is complete, we switch to the new index by connecting the
 index alias to it which becomes the new `primary` index. At the end, we resume the writes and typical operation resumes.
 
+### Using zero-downtime reindexing
+
+You can use zero-downtime reindexing to configure index settings or mappings that cannot be changed without creating a new index and copying existing data. You should not use zero-downtime reindexing to fix missing data. Zero-downtime reindexing does not add data to the search cluster if the data is not already indexed. You must complete all [advanced search migrations](#advanced-search-migrations) before you start reindexing.
+
 ### Trigger the reindex via the advanced search administration
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34069) in GitLab 13.2.
@@ -662,7 +666,7 @@ For basic guidance on choosing a cluster configuration you may refer to [Elastic
 
 - Generally, you want to use at least a 2-node cluster configuration with one replica, which allows you to have resilience. If your storage usage is growing quickly, you may want to plan horizontal scaling (adding more nodes) beforehand.
 - It's not recommended to use HDD storage with the search cluster, because it takes a hit on performance. It's better to use SSD storage (NVMe or SATA SSD drives for example).
-- You should not use [coordinating-only nodes](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-node.html#coordinating-only-node) with large instances. Coordinating-only nodes are smaller than [data nodes](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-node.html#data-node), which can impact performance and advanced search migrations.
+- You should not use [coordinating-only nodes](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-node.html#coordinating-only-node) with large instances. Coordinating-only nodes are smaller than [data nodes](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-node.html#data-node), which can impact performance and [advanced search migrations](#advanced-search-migrations).
 - You can use the [GitLab Performance Tool](https://gitlab.com/gitlab-org/quality/performance) to benchmark search performance with different search cluster sizes and configurations.
 - `Heap size` should be set to no more than 50% of your physical RAM. Additionally, it shouldn't be set to more than the threshold for zero-based compressed oops. The exact threshold varies, but 26 GB is safe on most systems, but can also be as large as 30 GB on some systems. See [Heap size settings](https://www.elastic.co/guide/en/elasticsearch/reference/current/important-settings.html#heap-size-settings) and [Setting JVM options](https://www.elastic.co/guide/en/elasticsearch/reference/current/jvm-options.html) for more details.
 - Number of CPUs (CPU cores) per node usually corresponds to the `Number of Elasticsearch shards` setting described below.