CE backport of "Configure the number of ES shards and replicas"

db/migrate/20190515125613_add_application_settings_elasticsearch_shards.rb

+# frozen_string_literal: true
+
+class AddApplicationSettingsElasticsearchShards < ActiveRecord::Migration[5.1]
+  DOWNTIME = false
+
+  def change
+    add_column :application_settings, :elasticsearch_shards, :integer, null: false, default: 5
+    add_column :application_settings, :elasticsearch_replicas, :integer, null: false, default: 1
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 20190506135400) do
+ActiveRecord::Schema.define(version: 20190515125613) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -189,6 +189,8 @@ ActiveRecord::Schema.define(version: 20190506135400) do
     t.string "encrypted_external_auth_client_key_pass_iv"
     t.string "lets_encrypt_notification_email"
     t.boolean "lets_encrypt_terms_of_service_accepted", default: false, null: false
+    t.integer "elasticsearch_shards", default: 5, null: false
+    t.integer "elasticsearch_replicas", default: 1, null: false
     t.index ["usage_stats_set_by_user_id"], name: "index_application_settings_on_usage_stats_set_by_user_id", using: :btree
   end

doc/integration/elasticsearch.md

@@ -131,6 +131,8 @@ The following Elasticsearch settings are available:
 | `Use the new repository indexer (beta)` | Perform repository indexing using [GitLab Elasticsearch Indexer](https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer). |
 | `Search with Elasticsearch enabled` | Enables/disables using Elasticsearch in search. |
 | `URL`                              | The URL to use for connecting to Elasticsearch. Use a comma-separated list to support clustering (e.g., "http://host1, https://host2:9200"). If your Elasticsearch instance is password protected, pass the `username:password` in the URL (e.g., `http://<username>:<password>@<elastic_host>:9200/`). |
+| `Number of Elasticsearch shards` | Elasticsearch indexes are split into multiple shards for performance reasons. In general, larger indexes need to have more shards. Changes to this value do not take effect until the index is recreated. You can read more about tradeoffs in the [Elasticsearch documentation](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-create-index.html#create-index-settings) |
+| `Number of Elasticsearch replicas` | Each Elasticsearch shard can have a number of replicas. These are a complete copy of the shard, and can provide increased query performance or resilience against hardware failure. Increasing this value will greatly increase total disk space required by the index. |
 | `Limit namespaces and projects that can be indexed` | Enabling this will allow you to select namespaces and projects to index. All other namespaces and projects will use database search instead. Please note that if you enable this option but do not select any namespaces or projects, none will be indexed. [Read more below](#limiting-namespaces-and-projects).
 | `Using AWS hosted Elasticsearch with IAM credentials` | Sign your Elasticsearch requests using [AWS IAM authorization](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_access-keys.html) or [AWS EC2 Instance Profile Credentials](http://docs.aws.amazon.com/codedeploy/latest/userguide/getting-started-create-iam-instance-profile.html#getting-started-create-iam-instance-profile-cli). The policies must be configured to allow `es:*` actions. |
 | `AWS Region` | The AWS region your Elasticsearch service is located in. |