diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
index 56e4f2d..ed97514 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
@@ -64,7 +64,7 @@ public static function open(array &$connection_options = array()) {
     // Character set is added to dsn to ensure PDO uses the proper character
     // set when escaping. This has security implications. See
     // https://www.drupal.org/node/1201452 for further discussion.
-    $dsn .= ';charset=utf8';
+    $dsn .= ';charset=utf8mb4';
     if (!empty($connection_options['database'])) {
       $dsn .= ';dbname=' . $connection_options['database'];
     }
@@ -92,13 +92,13 @@ public static function open(array &$connection_options = array()) {
     $pdo = new \PDO($dsn, $connection_options['username'], $connection_options['password'], $connection_options['pdo']);
 
     // Force MySQL to use the UTF-8 character set. Also set the collation, if a
-    // certain one has been set; otherwise, MySQL defaults to 'utf8_general_ci'
-    // for UTF-8.
+    // certain one has been set; otherwise, MySQL defaults to
+    // 'utf8mb4_general_ci' for utf8mb4.
     if (!empty($connection_options['collation'])) {
-      $pdo->exec('SET NAMES utf8 COLLATE ' . $connection_options['collation']);
+      $pdo->exec('SET NAMES utf8mb4 COLLATE ' . $connection_options['collation']);
     }
     else {
-      $pdo->exec('SET NAMES utf8');
+      $pdo->exec('SET NAMES utf8mb4');
     }
 
     // Set MySQL init_commands if not already defined.  Default Drupal's MySQL
diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
index 8237c79..6326bcf 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
@@ -87,7 +87,7 @@ protected function createTableSql($name, $table) {
     // Provide defaults if needed.
     $table += array(
       'mysql_engine' => 'InnoDB',
-      'mysql_character_set' => 'utf8',
+      'mysql_character_set' => 'utf8mb4',
     );
 
     $sql = "CREATE TABLE {" . $name . "} (\n";
@@ -108,8 +108,8 @@ protected function createTableSql($name, $table) {
 
     $sql .= 'ENGINE = ' . $table['mysql_engine'] . ' DEFAULT CHARACTER SET ' . $table['mysql_character_set'];
     // By default, MySQL uses the default collation for new tables, which is
-    // 'utf8_general_ci' for utf8. If an alternate collation has been set, it
-    // needs to be explicitly specified.
+    // 'utf8mb4_general_ci' for utf8mb4. If an alternate collation has been
+    // set, it needs to be explicitly specified.
     // @see DatabaseConnection_mysql
     if (!empty($info['collation'])) {
       $sql .= ' COLLATE ' . $info['collation'];
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/UriItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/UriItem.php
index 1afd100..54a3b7f 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/UriItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/UriItem.php
@@ -55,7 +55,7 @@ public static function schema(FieldStorageDefinitionInterface $field_definition)
     return array(
       'columns' => array(
         'value' => array(
-          'type' => 'varchar',
+          'type' => $field_definition->getSetting('is_ascii') === TRUE ? 'varchar_ascii' : 'varchar',
           'length' => (int) $field_definition->getSetting('max_length'),
           'binary' => $field_definition->getSetting('case_sensitive'),
         ),
diff --git a/core/modules/aggregator/src/Entity/Feed.php b/core/modules/aggregator/src/Entity/Feed.php
index a5ed081..c207276 100644
--- a/core/modules/aggregator/src/Entity/Feed.php
+++ b/core/modules/aggregator/src/Entity/Feed.php
@@ -145,7 +145,6 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       ->setLabel(t('Title'))
       ->setDescription(t('The name of the feed (or the name of the website providing the feed).'))
       ->setRequired(TRUE)
-      ->setSetting('max_length', 255)
       ->setDisplayOptions('form', array(
         'type' => 'string_textfield',
         'weight' => -5,
diff --git a/core/modules/aggregator/src/FeedStorageSchema.php b/core/modules/aggregator/src/FeedStorageSchema.php
index d251ed5..117c3fe 100644
--- a/core/modules/aggregator/src/FeedStorageSchema.php
+++ b/core/modules/aggregator/src/FeedStorageSchema.php
@@ -33,7 +33,7 @@ protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $st
           break;
 
         case 'title':
-          $this->addSharedTableFieldUniqueKey($storage_definition, $schema);
+          $this->addSharedTableFieldIndex($storage_definition, $schema);
           break;
       }
     }
diff --git a/core/modules/block_content/src/BlockContentStorageSchema.php b/core/modules/block_content/src/BlockContentStorageSchema.php
deleted file mode 100644
index c1674ae..0000000
--- a/core/modules/block_content/src/BlockContentStorageSchema.php
+++ /dev/null
@@ -1,52 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\block_content\BlockContentStorageSchema.
- */
-
-namespace Drupal\block_content;
-
-use Drupal\Core\Entity\ContentEntityTypeInterface;
-use Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema;
-use Drupal\Core\Field\FieldStorageDefinitionInterface;
-
-/**
- * Defines the block content schema handler.
- */
-class BlockContentStorageSchema extends SqlContentEntityStorageSchema {
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function getEntitySchema(ContentEntityTypeInterface $entity_type, $reset = FALSE) {
-    $schema = parent::getEntitySchema($entity_type, $reset);
-
-    $schema['block_content_field_data']['unique keys'] += array(
-      'block_content__info' => array('info', 'langcode'),
-    );
-
-    return $schema;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $storage_definition, $table_name, array $column_mapping) {
-    $schema = parent::getSharedTableFieldSchema($storage_definition, $table_name, $column_mapping);
-    $field_name = $storage_definition->getName();
-
-    if ($table_name == 'block_content_field_data') {
-      switch ($field_name) {
-        case 'info':
-          // Improves the performance of the block_content__info index defined
-          // in getEntitySchema().
-          $schema['fields'][$field_name]['not null'] = TRUE;
-          break;
-      }
-    }
-
-    return $schema;
-  }
-
-}
diff --git a/core/modules/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
index 046640c..76fe365 100644
--- a/core/modules/block_content/src/Entity/BlockContent.php
+++ b/core/modules/block_content/src/Entity/BlockContent.php
@@ -22,7 +22,7 @@
  *   bundle_label = @Translation("Custom block type"),
  *   handlers = {
  *     "storage" = "Drupal\Core\Entity\Sql\SqlContentEntityStorage",
- *     "storage_schema" = "Drupal\block_content\BlockContentStorageSchema",
+ *     "storage_schema" = "Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema",
  *     "access" = "Drupal\block_content\BlockContentAccessControlHandler",
  *     "list_builder" = "Drupal\block_content\BlockContentListBuilder",
  *     "view_builder" = "Drupal\block_content\BlockContentViewBuilder",
diff --git a/core/modules/file/src/Entity/File.php b/core/modules/file/src/Entity/File.php
index ff38ab3..92e86cd 100644
--- a/core/modules/file/src/Entity/File.php
+++ b/core/modules/file/src/Entity/File.php
@@ -249,6 +249,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
     $fields['uri'] = BaseFieldDefinition::create('uri')
       ->setLabel(t('URI'))
       ->setDescription(t('The URI to access the file (either local or remote).'))
+      ->setSetting('is_ascii', TRUE)
       ->setSetting('max_length', 255)
       ->setSetting('case_sensitive', TRUE);
 
diff --git a/core/modules/file/src/FileStorage.php b/core/modules/file/src/FileStorage.php
index 872435f..ad342c2 100644
--- a/core/modules/file/src/FileStorage.php
+++ b/core/modules/file/src/FileStorage.php
@@ -8,6 +8,7 @@
 namespace Drupal\file;
 
 use Drupal\Core\Entity\Sql\SqlContentEntityStorage;
+use Drupal\Core\Entity\EntityInterface;
 
 /**
  * File storage for files.
@@ -17,6 +18,39 @@ class FileStorage extends SqlContentEntityStorage implements FileStorageInterfac
   /**
    * {@inheritdoc}
    */
+  protected function doLoadMultiple(array $ids = NULL) {
+    $entities = parent::doLoadMultiple($ids);
+    foreach ($entities as $entity) {
+      // @see FileStorage::doSave()
+      $entity->setFileUri($this->uriFromAscii($entity->getFileUri()));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function loadByProperties(array $values = array()) {
+    foreach ($values as $name => $value) {
+      if ($name == 'uri') {
+        // @see FileStorage::doSave()
+        $values[$name] = $this->uriToAscii($value);
+      }
+    }
+    parent::loadByProperties($values);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function doSave($id, EntityInterface $entity) {
+    $entity->setFileUri($this->uriToAscii($entity->getFileUri()));
+    parent::doSave($id, $entity);
+    $entity->setFileUri($this->uriFromAscii($entity->getFileUri()));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function spaceUsed($uid = NULL, $status = FILE_STATUS_PERMANENT) {
     $query = $this->database->select($this->entityType->getBaseTable(), 'f')
       ->condition('f.status', $status);
@@ -27,4 +61,33 @@ public function spaceUsed($uid = NULL, $status = FILE_STATUS_PERMANENT) {
     return $query->execute()->fetchField();
   }
 
+  /**
+   * Turns a URI string into an ASCII encoded string.
+   *
+   * This will be used to save non-ASCII data into the URI field, which is
+   * marked as an ASCII field so it can contain a unique constraint and
+   * be 255 characters long at the same time.
+   *
+   * @param string $url
+   *   Provided URI, possibly non-ASCII.
+   *
+   * @return string
+   *   ASCII-encoded URI.
+   */
+  protected function uriToAscii($url) {
+    return str_replace('%3A', ':', str_replace('%2F', '/', rawurlencode($url)));
+  }
+
+  /**
+   * Inverse method of FileStorage::urlToAscii().
+   *
+   * @param string $url
+   *   ASCII encoded URI, from database.
+   *
+   * @return string
+   *   Originally provided URI.
+   */
+  protected function uriFromAscii($url) {
+    return rawurldecode($url);
+  }
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
index 435907b..b30679c 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -261,6 +261,12 @@ protected function ensureTables() {
       foreach ($this->migration->getSourcePlugin()->getIds() as $id_definition) {
         $mapkey = 'sourceid' . $count++;
         $source_id_schema[$mapkey] = $this->getFieldSchema($id_definition);
+
+        // With InnoDB, utf8mb4-based primary keys can't be over 191 characters.
+        // Use ASCII-based primary keys instead.
+        if (isset($source_id_schema[$mapkey]['type']) && $source_id_schema[$mapkey]['type'] == 'varchar') {
+          $source_id_schema[$mapkey]['type'] = 'varchar_ascii';
+        }
         $pks[] = $mapkey;
       }
 
diff --git a/core/modules/migrate_drupal/src/Tests/Table/d6/System.php b/core/modules/migrate_drupal/src/Tests/Table/d6/System.php
index ae42df9..cfb1ab5 100644
--- a/core/modules/migrate_drupal/src/Tests/Table/d6/System.php
+++ b/core/modules/migrate_drupal/src/Tests/Table/d6/System.php
@@ -6,7 +6,7 @@
  *
  * THIS IS A GENERATED FILE. DO NOT EDIT.
  *
- * @see cores/scripts/dump-database-d6.sh
+ * @see core/scripts/dump-database-d6.sh
  * @see https://www.drupal.org/sandbox/benjy/2405029
  */
 
@@ -26,7 +26,7 @@ public function load() {
       ),
       'fields' => array(
         'filename' => array(
-          'type' => 'varchar',
+          'type' => 'varchar_ascii',
           'not null' => TRUE,
           'length' => '255',
           'default' => '',
diff --git a/core/modules/node/src/Tests/NodeViewTest.php b/core/modules/node/src/Tests/NodeViewTest.php
index 2281dd8..c60d44a 100644
--- a/core/modules/node/src/Tests/NodeViewTest.php
+++ b/core/modules/node/src/Tests/NodeViewTest.php
@@ -33,4 +33,16 @@ public function testHtmlHeadLinks() {
     $this->assertEqual($result[0]['href'], $node->url());
   }
 
+  /**
+   * Tests that we store and retrieve multi-byte UTF-8 characters correctly.
+   */
+  public function testMultiByteUtf8() {
+    $title = '🐝';
+    $this->assertTrue(mb_strlen($title, 'utf-8') < strlen($title), 'Title has multi-byte characters.');
+    $node = $this->drupalCreateNode(array('title' => $title));
+    $this->drupalGet($node->urlInfo());
+    $result = $this->xpath('//span[contains(@class, "field-name-title")]');
+    $this->assertEqual((string) $result[0], $title, 'The passed title was returned.');
+  }
+
 }
diff --git a/core/modules/system/src/Tests/Database/RegressionTest.php b/core/modules/system/src/Tests/Database/RegressionTest.php
index 81f962d..94c98a5 100644
--- a/core/modules/system/src/Tests/Database/RegressionTest.php
+++ b/core/modules/system/src/Tests/Database/RegressionTest.php
@@ -26,15 +26,15 @@ class RegressionTest extends DatabaseTestBase {
    */
   function testRegression_310447() {
     // That's a 255 character UTF-8 string.
-    $name = str_repeat("é", 255);
+    $job = str_repeat("é", 255);
     db_insert('test')
       ->fields(array(
-        'name' => $name,
+        'name' => $this->randomMachineName(),
         'age' => 20,
-        'job' => 'Dancer',
+        'job' => $job,
       ))->execute();
 
-    $from_database = db_query('SELECT name FROM {test} WHERE name = :name', array(':name' => $name))->fetchField();
+    $from_database = db_query('SELECT job FROM {test} WHERE job = :job', array(':job' => $job))->fetchField();
     $this->assertIdentical($name, $from_database, 'The database handles UTF-8 characters cleanly.');
   }
 
diff --git a/core/modules/system/src/Tests/Database/SchemaTest.php b/core/modules/system/src/Tests/Database/SchemaTest.php
index 9a234ed..df1b0b6 100644
--- a/core/modules/system/src/Tests/Database/SchemaTest.php
+++ b/core/modules/system/src/Tests/Database/SchemaTest.php
@@ -72,7 +72,7 @@ function testSchema() {
       $columns = db_query('SHOW FULL COLUMNS FROM {test_table}');
       foreach ($columns as $column) {
         if ($column->Field == 'test_field_string') {
-          $string_check = ($column->Collation == 'utf8_general_ci');
+          $string_check = ($column->Collation == 'utf8mb4_general_ci');
         }
         if ($column->Field == 'test_field_string_ascii') {
           $string_ascii_check = ($column->Collation == 'ascii_general_ci');
diff --git a/core/modules/system/tests/modules/database_test/database_test.install b/core/modules/system/tests/modules/database_test/database_test.install
index 7c74c1c..cfd72d5 100644
--- a/core/modules/system/tests/modules/database_test/database_test.install
+++ b/core/modules/system/tests/modules/database_test/database_test.install
@@ -24,7 +24,7 @@ function database_test_schema() {
       ),
       'name' => array(
         'description' => "A person's name",
-        'type' => 'varchar',
+        'type' => 'varchar_ascii',
         'length' => 255,
         'not null' => TRUE,
         'default' => '',
@@ -75,7 +75,7 @@ function database_test_schema() {
       ),
       'job' => array(
         'description' => "The person's job",
-        'type' => 'varchar',
+        'type' => 'varchar_ascii',
         'length' => 255,
         'not null' => TRUE,
         'default' => '',
@@ -106,7 +106,7 @@ function database_test_schema() {
       ),
       'job' => array(
         'description' => "The person's job",
-        'type' => 'varchar',
+        'type' => 'varchar_ascii',
         'length' => 255,
         'not null' => TRUE,
         'default' => '',
@@ -197,7 +197,7 @@ function database_test_schema() {
       ),
       'name' => array(
         'description' => "A person's name.",
-        'type' => 'varchar',
+        'type' => 'varchar_ascii',
         'length' => 255,
         'not null' => FALSE,
         'default' => '',
@@ -228,7 +228,7 @@ function database_test_schema() {
       ),
       'name' => array(
         'description' => "A person's name.",
-        'type' => 'varchar',
+        'type' => 'varchar_ascii',
         'length' => 255,
         'not null' => FALSE,
         'default' => '',
diff --git a/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php b/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php
index 7e057d4..d6b6c59 100644
--- a/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php
+++ b/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php
@@ -45,7 +45,10 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
     $fields['id'] = BaseFieldDefinition::create('string')
       ->setLabel(t('ID'))
       ->setDescription(t('The ID of the test entity.'))
-      ->setReadOnly(TRUE);
+      ->setReadOnly(TRUE)
+      // In order to work around the InnoDB 191 character limit on utf8mb4
+      // primary keys, we set the character set for the field to ASCII.
+      ->setSetting('is_ascii', TRUE);
     return $fields;
   }
 
diff --git a/core/modules/user/src/UserStorageSchema.php b/core/modules/user/src/UserStorageSchema.php
index 447469d..6247b10 100644
--- a/core/modules/user/src/UserStorageSchema.php
+++ b/core/modules/user/src/UserStorageSchema.php
@@ -52,6 +52,9 @@ protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $st
           // Improves the performance of the user__name index defined
           // in getEntitySchema().
           $schema['fields'][$field_name]['not null'] = TRUE;
+          // Make sure the field is no longer than 191 characters so we can
+          // add a unique constraint in MySQL.
+          $schema['fields'][$field_name]['length'] = USERNAME_MAX_LENGTH;
           break;
 
         case 'mail':
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index d175edf..e55eb56 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -25,6 +25,8 @@
 
 /**
  * Maximum length of username text field.
+ *
+ * Keep this under 191 characters so we can use a unique constraint in MySQL.
  */
 const USERNAME_MAX_LENGTH = 60;
 
diff --git a/core/modules/views/src/Tests/ViewTestData.php b/core/modules/views/src/Tests/ViewTestData.php
index 4b5340e..34fcd44 100644
--- a/core/modules/views/src/Tests/ViewTestData.php
+++ b/core/modules/views/src/Tests/ViewTestData.php
@@ -75,7 +75,7 @@ public static function schemaDefinition() {
         ),
         'name' => array(
           'description' => "A person's name",
-          'type' => 'varchar',
+          'type' => 'varchar_ascii',
           'length' => 255,
           'not null' => TRUE,
           'default' => '',
diff --git a/core/scripts/dump-database-d6.sh b/core/scripts/dump-database-d6.sh
index ace93d9..85309fd 100644
--- a/core/scripts/dump-database-d6.sh
+++ b/core/scripts/dump-database-d6.sh
@@ -53,6 +53,11 @@
 $schema = drupal_get_schema();
 ksort($schema);
 
+// Override the field type of the filename primary key to bypass the
+// InnoDB 191 character limitation.
+if ($schema['system']['primary key'] == 'filename' && $schema['system']['fields']['filename']['type'] == 'varchar') {
+  $schema['system']['fields']['filename']['type'] = 'varchar_ascii';
+}
 // Export all the tables in the schema.
 foreach ($schema as $table => $data) {
   // Remove descriptions to save time and code.
diff --git a/core/tests/Drupal/Tests/Core/Routing/RoutingFixtures.php b/core/tests/Drupal/Tests/Core/Routing/RoutingFixtures.php
index ffbfb03..f0aba22 100644
--- a/core/tests/Drupal/Tests/Core/Routing/RoutingFixtures.php
+++ b/core/tests/Drupal/Tests/Core/Routing/RoutingFixtures.php
@@ -176,7 +176,7 @@ public function routingTableDefinition() {
       'fields' => array(
         'name' => array(
           'description' => 'Primary Key: Machine name of this route',
-          'type' => 'varchar',
+          'type' => 'varchar_ascii',
           'length' => 255,
           'not null' => TRUE,
           'default' => '',
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 3783d0a..f9a5a1b 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -75,7 +75,7 @@
  *   'host' => 'localhost',
  *   'port' => 3306,
  *   'prefix' => 'myprefix_',
- *   'collation' => 'utf8_general_ci',
+ *   'collation' => 'utf8mb4_general_ci',
  * );
  * @endcode
  *
@@ -127,7 +127,7 @@
  *   'password' => 'password',
  *   'host' => 'localhost',
  *   'prefix' => 'main_',
- *   'collation' => 'utf8_general_ci',
+ *   'collation' => 'utf8mb4_general_ci',
  * );
  * @endcode
  *