diff --git a/core/lib/Drupal/Core/Entity/Sql/DefaultTableMapping.php b/core/lib/Drupal/Core/Entity/Sql/DefaultTableMapping.php
index 7555b45..0f91c66 100644
--- a/core/lib/Drupal/Core/Entity/Sql/DefaultTableMapping.php
+++ b/core/lib/Drupal/Core/Entity/Sql/DefaultTableMapping.php
@@ -186,8 +186,8 @@ public function initializeMapping() {
     }
     elseif ($revisionable && $translatable) {
       // The revisionable multilingual layout stores key field values in the
-      // base table, except for language, which is stored in the revision
-      // table along with revision metadata. The revision data table holds
+      // base table and the revision table holds the entity ID, revision ID and
+      // langcode ID along with revision metadata. The revision data table holds
       // data field values for all the revisionable fields and the data table
       // holds the data field values for all non-revisionable fields. The data
       // field values of revisionable fields are denormalized in the data
diff --git a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
index ad94f46..fd5ac4b 100644
--- a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
@@ -405,6 +405,22 @@ public function onFieldStorageDefinitionUpdate(FieldStorageDefinitionInterface $
    * {@inheritdoc}
    */
   public function onFieldStorageDefinitionDelete(FieldStorageDefinitionInterface $storage_definition) {
+    // Only configurable fields currently support purging, so prevent deletion
+    // of ones we can't purge if they have existing data.
+    // @todo Add purging to all fields: https://www.drupal.org/node/2282119.
+    try {
+      if (!($storage_definition instanceof FieldStorageConfigInterface) && $this->storage->countFieldData($storage_definition, TRUE)) {
+        throw new FieldStorageDefinitionUpdateForbiddenException('Unable to delete a field (' . $storage_definition->getName() . ' in ' . $storage_definition->getTargetEntityTypeId() . ' entity) with data that cannot be purged.');
+      }
+    }
+    catch (DatabaseException $e) {
+      // This may happen when changing field storage schema, since we are not
+      // able to use a table mapping matching the passed storage definition.
+      // @todo Revisit this once we are able to instantiate the table mapping
+      //   properly. See https://www.drupal.org/node/2274017.
+      return;
+    }
+
     // Retrieve a table mapping which contains the deleted field still.
     $table_mapping = $this->storage->getTableMapping(
       $this->entityManager->getLastInstalledFieldStorageDefinitions($this->entityType->id())