Default to zero translog retention

Now that elastic#45136 means we perform recoveries from the index rather than the
translog (if soft deletes are enabled) there is no need to retain extra
translog for performing peer recoveries. This commit reduces the default
translog retention to zero so that it can be discarded more quickly.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/index/IndexSettings.java

@@ -195,24 +195,6 @@ public final class IndexSettings {
             new ByteSizeValue(Long.MAX_VALUE, ByteSizeUnit.BYTES),
             Property.Dynamic, Property.IndexScope);
 
-    /**
-     * Controls how long translog files that are no longer needed for persistence reasons
-     * will be kept around before being deleted. A longer retention policy is useful to increase
-     * the chance of ops based recoveries.
-     **/
-    public static final Setting<TimeValue> INDEX_TRANSLOG_RETENTION_AGE_SETTING =
-        Setting.timeSetting("index.translog.retention.age", TimeValue.timeValueHours(12), TimeValue.timeValueMillis(-1),
-            Property.Dynamic, Property.IndexScope);
-
-    /**
-     * Controls how many translog files that are no longer needed for persistence reasons
-     * will be kept around before being deleted. Keeping more files is useful to increase
-     * the chance of ops based recoveries.
-     **/
-    public static final Setting<ByteSizeValue> INDEX_TRANSLOG_RETENTION_SIZE_SETTING =
-        Setting.byteSizeSetting("index.translog.retention.size", new ByteSizeValue(512, ByteSizeUnit.MB), Property.Dynamic,
-            Property.IndexScope);
-
     /**
      * The maximum size of a translog generation. This is independent of the maximum size of
      * translog operations that have not been flushed.
@@ -258,6 +240,23 @@ public final class IndexSettings {
         Setting.longSetting("index.soft_deletes.retention.operations", 0, 0,
             Property.IndexScope, Property.Dynamic);
 
+    /**
+     * Controls how long translog files that are no longer needed for persistence reasons
+     * will be kept around before being deleted.
+     **/
+    public static final Setting<TimeValue> INDEX_TRANSLOG_RETENTION_AGE_SETTING =
+        Setting.timeSetting("index.translog.retention.age",
+            settings -> INDEX_SOFT_DELETES_SETTING.get(settings) ? TimeValue.ZERO : TimeValue.timeValueHours(12), TimeValue.MINUS_ONE,
+            Property.Dynamic, Property.IndexScope);
+
+    /**
+     * Controls how many translog files that are no longer needed for persistence reasons
+     * will be kept around before being deleted.
+     **/
+    public static final Setting<ByteSizeValue> INDEX_TRANSLOG_RETENTION_SIZE_SETTING =
+        Setting.byteSizeSetting("index.translog.retention.size", settings -> INDEX_SOFT_DELETES_SETTING.get(settings) ? "0b" : "512mb",
+            Property.Dynamic, Property.IndexScope);
+
     /**
      * Controls the maximum length of time since a retention lease is created or renewed before it is considered expired.
      */