diff --git a/inc/Engine/Common/Cache/CacheInterface.php b/inc/Engine/Common/Cache/CacheInterface.php
index 5daeb7b9dc..bdc8c95f6e 100644
--- a/inc/Engine/Common/Cache/CacheInterface.php
+++ b/inc/Engine/Common/Cache/CacheInterface.php
@@ -33,4 +33,13 @@ public function get_root_path(): string;
 	 * @return string
 	 */
 	public function generate_path( string $url ): string;
+
+	/**
+	 * Wipes the whole cache directory.
+	 *
+	 * @param array $preserve_dirs List of directories to be preserved.
+	 *
+	 * @return bool True on success and false on failure.
+	 */
+	public function full_clear( array $preserve_dirs = [] ): bool;
 }
diff --git a/inc/Engine/Common/Cache/FilesystemCache.php b/inc/Engine/Common/Cache/FilesystemCache.php
index 7096898174..d3c0d43478 100644
--- a/inc/Engine/Common/Cache/FilesystemCache.php
+++ b/inc/Engine/Common/Cache/FilesystemCache.php
@@ -103,6 +103,32 @@ public function clear() {
 		return true;
 	}
 
+	/**
+	 * Clear the whole background-css directory.
+	 *
+	 * @param array $preserve_dirs List of directories to be preserved.
+	 *
+	 * @return bool True on success and false on failure.
+	 */
+	public function full_clear( array $preserve_dirs = [] ): bool {
+		$base_path = $this->get_base_path();
+		if ( ! $this->filesystem->exists( $base_path ) ) {
+			return false;
+		}
+
+		if ( ! empty( $preserve_dirs ) ) {
+			$preserve_dirs = array_map(
+					function ( $dir ) use ( $base_path ) {
+						return $base_path . $dir;
+					},
+				$preserve_dirs
+				);
+		}
+
+		rocket_rrmdir( $base_path, $preserve_dirs, $this->filesystem );
+		return true;
+	}
+
 	/**
 	 * Obtains multiple cache items by their unique keys.
 	 *
@@ -226,11 +252,20 @@ public function is_accessible(): bool {
 	}
 
 	/**
-	 * Get root path from the cache.
+	 * Get root path from the cache (including current blog ID) with trailing slash.
 	 *
 	 * @return string
 	 */
 	public function get_root_path(): string {
+		return $this->get_base_path() . get_current_blog_id() . '/';
+	}
+
+	/**
+	 * Get base path from the cache (/cache/background-css/) with trailing slash.
+	 *
+	 * @return string
+	 */
+	private function get_base_path(): string {
 		return rtrim( _rocket_normalize_path( rocket_get_constant( 'WP_ROCKET_CACHE_ROOT_PATH' ) ) . $this->root_folder, '/' ) . '/';
 	}
 }
diff --git a/inc/Engine/Common/ExtractCSS/ServiceProvider.php b/inc/Engine/Common/ExtractCSS/ServiceProvider.php
index 0148a2f9cc..dea19f7be0 100644
--- a/inc/Engine/Common/ExtractCSS/ServiceProvider.php
+++ b/inc/Engine/Common/ExtractCSS/ServiceProvider.php
@@ -41,7 +41,7 @@ public function register(): void {
 		 *
 		 * @param string $root Background CSS cache folder.
 		 */
-		$root = apply_filters( 'rocket_lazyload_css_cache_root', 'background-css' );
+		$root = apply_filters( 'rocket_lazyload_css_cache_root', 'background-css/' );
 		$this->getContainer()->add( 'lazyload_css_cache', FilesystemCache::class )
 			->addArgument( $root );
 		$this->getContainer()->addShared( 'common_extractcss_subscriber', Subscriber::class );
diff --git a/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber.php b/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber.php
index 525608e7c1..cfc265e103 100644
--- a/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber.php
+++ b/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber.php
@@ -34,6 +34,7 @@ public static function get_subscribed_events() {
 			'rocket_meta_boxes_fields'       => [ 'add_meta_box', 8 ],
 			'admin_notices'                  => 'maybe_add_error_notice',
 			'rocket_safe_mode_reset_options' => 'add_option_safemode',
+			'wp_rocket_upgrade'              => [ 'clear_background_css_with_upgrade', 10, 2 ],
 		];
 	}
 
@@ -79,4 +80,22 @@ public function add_option_safemode( array $options ) {
 		$options['lazyload_css_bg_img'] = 0;
 		return $options;
 	}
+
+	/**
+	 * Upgrade callback.
+	 *
+	 * @param string $new_version Plugin new version.
+	 * @param string $old_version Plugin old version.
+	 * @return void
+	 */
+	public function clear_background_css_with_upgrade( $new_version, $old_version ) {
+		if ( empty( $old_version ) || version_compare( $old_version, '3.18', '>' ) ) {
+			return;
+		}
+
+		$preserve_dirs = is_multisite() ? get_sites( [ 'fields' => 'ids' ] ) : [ get_current_blog_id() ];
+
+		// Completely clear background-css directory.
+		$this->cache->full_clear( $preserve_dirs );
+	}
 }
diff --git a/inc/Engine/Media/Lazyload/CSS/ServiceProvider.php b/inc/Engine/Media/Lazyload/CSS/ServiceProvider.php
index 10f0189942..e0bbc28a15 100644
--- a/inc/Engine/Media/Lazyload/CSS/ServiceProvider.php
+++ b/inc/Engine/Media/Lazyload/CSS/ServiceProvider.php
@@ -24,7 +24,6 @@ class ServiceProvider extends AbstractServiceProvider {
 	 * @var array
 	 */
 	protected $provides = [
-		'lazyload_css_cache',
 		'lazyload_css_subscriber',
 	];
 
@@ -45,9 +44,6 @@ public function provides( string $id ): bool {
 	 * @return void
 	 */
 	public function register(): void {
-		$this->getContainer()->add( 'lazyload_css_cache', FilesystemCache::class )
-			->addArgument( apply_filters( 'rocket_lazyload_css_cache_root', 'background-css/' . get_current_blog_id() ) );
-
 		$cache = $this->getContainer()->get( 'lazyload_css_cache' );
 
 		$this->getContainer()->add( 'lazyload_css_context', LazyloadCSSContext::class )
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/clear.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/clear.php
index 9c6d6cd7e4..da017b8f25 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/clear.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/clear.php
@@ -7,7 +7,7 @@
 		],
         'expected' => [
 			'output' => true,
-			'path' => '/cache/background-css/'
+			'path' => '/cache/background-css/1/'
 		]
     ],
 	'notExistsShouldReturnFalse' => [
@@ -17,7 +17,7 @@
 		],
 		'expected' => [
 			'output' => true,
-			'path' => '/cache/background-css/'
+			'path' => '/cache/background-css/1/'
 		]
 	],
 
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/delete.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/delete.php
index 70d2d2ca1b..a550a680c4 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/delete.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/delete.php
@@ -13,7 +13,7 @@
 			  ]
 		],
         'expected' => [
-			'path' => '/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => true
 		]
     ],
@@ -30,7 +30,7 @@
 			]
 		],
 		'expected' => [
-			'path' => '/cache/background-css/example.org/blog/test',
+			'path' => '/cache/background-css/1/example.org/blog/test',
 			'output' => true
 		]
 	],
@@ -47,7 +47,7 @@
 			]
 		],
 		'expected' => [
-			'path' => '/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => false
 		]
 	],
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php
index 493fbdf28a..3ddf7ecc16 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php
@@ -19,12 +19,12 @@
 				],
 			],
 			'is_dir' => [
-				'/cache/background-css/example.org/blog/test/file.css' => false,
-				'/cache/background-css/example.org/blog/test' => true,
+				'/cache/background-css/1/example.org/blog/test/file.css' => false,
+				'/cache/background-css/1/example.org/blog/test' => true,
 			],
 			'exists' => [
-				'/cache/background-css/example.org/blog/test/file.css' => true,
-				'/cache/background-css/example.org/blog/test' => true,
+				'/cache/background-css/1/example.org/blog/test/file.css' => true,
+				'/cache/background-css/1/example.org/blog/test' => true,
 			],
 		],
         'expected' => [
@@ -50,11 +50,11 @@
 				],
 			],
 			'is_dir' => [
-				'/cache/background-css/example.org/blog/test/file.css' => false,
+				'/cache/background-css/1/example.org/blog/test/file.css' => false,
 			],
 			'exists' => [
-				'/cache/background-css/example.org/blog/test/file.css' => true,
-				'/cache/background-css/example.org/blog/test' => false,
+				'/cache/background-css/1/example.org/blog/test/file.css' => true,
+				'/cache/background-css/1/example.org/blog/test' => false,
 			],
 		],
 		'expected' => [
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php
index 5c023e121f..bbe2cd21c5 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php
@@ -19,8 +19,8 @@
         ],
         'expected' => [
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
-			'output' => 'http://example.org/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
+			'output' => 'http://example.org/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'home_url' => 'http://example.org',
 		]
     ],
@@ -43,8 +43,8 @@
 		],
 		'expected' => [
 			'url' => '/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
-			'output' => 'http://example.org/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
+			'output' => 'http://example.org/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'home_url' => 'http://example.org',
 		]
 	],
@@ -67,8 +67,8 @@
 		],
 		'expected' => [
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
-			'output' => 'http://example.org/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
+			'output' => 'http://example.org/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'home_url' => 'http://example.org',
 		]
 	],
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/get.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/get.php
index eaf7b77d96..396f93ca40 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/get.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/get.php
@@ -15,7 +15,7 @@
 		],
         'expected' => [
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => 'content'
         ]
     ],
@@ -34,7 +34,7 @@
 		],
 		'expected' => [
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => null
 		]
 	]
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php
index f7e10bfb68..8794bf7fcf 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php
@@ -20,11 +20,11 @@
 			'root' => '/var/html/wp-content/cache',
 			'home_url' => 'http://example.org',
 			'exists' => [
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file.css' => true,
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file2.css' => false,
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css' => true,
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file2.css' => false,
 			],
 			'content' => [
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file.css' => 'content'
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css' => 'content'
 			]
         ],
         'expected' => [
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php
index d25635979f..4d185f0b5e 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php
@@ -4,7 +4,7 @@
         'config' => [
 			'root' => '/var/html/wp-content/cache',
         ],
-        'expected' => '/var/html/wp-content/cache/background-css/'
+        'expected' => '/var/html/wp-content/cache/background-css/1/'
     ],
 
 ];
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/has.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/has.php
index 168bca8c81..610ebe2b8d 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/has.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/has.php
@@ -13,7 +13,7 @@
 		],
         'expected' => [
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => true
         ]
     ],
@@ -30,7 +30,7 @@
 		],
 		'expected' => [
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => false
 		]
 	],
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php
index 0ae657da78..60d24c114a 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php
@@ -7,7 +7,7 @@
 			'root' => '/var/html/wp-content/cache'
         ],
         'expected' => [
-			'path' => '/var/html/wp-content/cache/background-css/',
+			'path' => '/var/html/wp-content/cache/background-css/1/',
 			'output' => false
         ]
     ],
@@ -19,7 +19,7 @@
 			'root' => '/var/html/wp-content/cache'
 		],
 		'expected' => [
-			'path' => '/var/html/wp-content/cache/background-css/',
+			'path' => '/var/html/wp-content/cache/background-css/1/',
 			'output' => true
 		]
 	],
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/set.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/set.php
index f352ff2252..10d9ef868e 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/set.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/set.php
@@ -17,7 +17,7 @@
         'expected' => [
 			'content' => 'content',
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => true,
         ]
     ],
@@ -38,7 +38,7 @@
 		'expected' => [
 			'content' => 'content',
 			'url' => 'http://example.org/blog/test/file.css',
-			'path' => '/var/html/wp-content/cache/background-css/example.org/blog/test/file.css',
+			'path' => '/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css',
 			'output' => false,
 		]
 	],
diff --git a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php
index 82ebcb1002..f064e16098 100644
--- a/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php
+++ b/tests/Fixtures/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php
@@ -21,11 +21,11 @@
 			'home_url' => 'http://example.org',
 			'root' => '/var/html/wp-content/cache',
 			'saved' => [
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file.css' => [
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css' => [
 					'content' => 'content',
 					'output' => true,
 				],
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file2.css' => [
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file2.css' => [
 					'content' => 'content2',
 					'output' => true,
 				],
@@ -56,11 +56,11 @@
 			'home_url' => 'http://example.org',
 			'root' => '/var/html/wp-content/cache',
 			'saved' => [
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file.css' => [
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file.css' => [
 					'content' => 'content',
 					'output' => true,
 				],
-				'/var/html/wp-content/cache/background-css/example.org/blog/test/file2.css' => [
+				'/var/html/wp-content/cache/background-css/1/example.org/blog/test/file2.css' => [
 					'content' => 'content2',
 					'output' => false,
 				],
diff --git a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber/maybeAddErrorNoticeIntegration.php b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber/maybeAddErrorNoticeIntegration.php
index 2fe6167cdf..f728883d6d 100644
--- a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber/maybeAddErrorNoticeIntegration.php
+++ b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Admin/Subscriber/maybeAddErrorNoticeIntegration.php
@@ -7,7 +7,7 @@
 WP Rocket</strong>
 cannot configure itself due to missing writing permissions.<br>
 Affected file/folder:<code>
-vfs://public/wp-content/cache/background-css/</code>
+vfs://public/wp-content/cache/background-css/1/</code>
 <br>
 Troubleshoot:<a href="https://docs.wp-rocket.me/article/626-how-to-make-system-files-htaccess-wp-config-writeable/?utm_source=wp_plugin&utm_medium=wp_rocket" target="_blank">
 How to make system files writeable</a>
diff --git a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_excluded.php b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_excluded.php
index 7bf875ada2..9a9e903221 100644
--- a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_excluded.php
+++ b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_excluded.php
@@ -7,9 +7,9 @@
 	<title>
 		HTML 5 Boilerplate</title>
 	<!-- external CSS background images here min and non-min file -->
-	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css?test=1&wpr_t=17895120">
+	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css?test=1&wpr_t=17895120">
 	<link rel="stylesheet" href="/wp-content/rocket-test-data/styles/excluded.css">
-	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css?wpr_t=17895120">
+	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css?wpr_t=17895120">
 	<link rel="stylesheet" href="https://new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css">
 	<!-- CSS loaded by js, probably won't be processed -->
 	<script>
diff --git a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_output.php b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_output.php
index 6a4d24e2ba..c454b8d207 100644
--- a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_output.php
+++ b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/HTML/html_output.php
@@ -6,9 +6,9 @@
 	<meta http-equiv="X-UA-Compatible" content="ie=edge">
 	<title>HTML 5 Boilerplate</title>
 	<!-- external CSS background images here min and non-min file -->
-	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css?test=1&wpr_t=17895120">
-	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/excluded.css?wpr_t=17895120">
-	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css?wpr_t=17895120">
+	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css?test=1&wpr_t=17895120">
+	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/excluded.css?wpr_t=17895120">
+	<link rel="stylesheet" href="http://example.org/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css?wpr_t=17895120">
 	<link rel="stylesheet" href="https://new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css">
 
 	<!-- CSS loaded by js, probably won't be processed -->
diff --git a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/maybeReplaceCssImages.php b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/maybeReplaceCssImages.php
index f72957137d..ecc5f19221 100644
--- a/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/maybeReplaceCssImages.php
+++ b/tests/Fixtures/inc/Engine/Media/Lazyload/CSS/Subscriber/integration/maybeReplaceCssImages.php
@@ -50,26 +50,26 @@
 			'expected' => [
 				'output' => $html_filtered,
 				'files' => [
-					'/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css' => [
+					'/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/lazyloaded.css')
 					],
-					'/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css.json' => [
+					'/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css.json' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/lazyload.css.json')
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/external_lazyloaded.css')
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css.json' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css.json' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/external.css.json')
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css' => [
 						'exists' => false,
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css.json' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css.json' => [
 						'exists' => false,
 					],
 				]
@@ -110,26 +110,26 @@
 			'expected' => [
 				'output' => $html_excluded,
 				'files' => [
-					'/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css' => [
+					'/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/lazyloaded.css')
 					],
-					'/wp-content/cache/background-css/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css.json' => [
+					'/wp-content/cache/background-css/1/example.org/wp-content/rocket-test-data/styles/lazyload_css_background_images.css.json' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/lazyload.css.json')
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/external_lazyloaded.css')
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css.json' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/lazyload_css_background_images.min.css.json' => [
 						'exists' => true,
 						'content' => file_get_contents(__DIR__ . '/CSS/external.css.json')
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css' => [
 						'exists' => false,
 					],
-					'/wp-content/cache/background-css/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css.json' => [
+					'/wp-content/cache/background-css/1/new.rocketlabsqa.ovh/wp-content/rocket-test-data/styles/no_background.css.json' => [
 						'exists' => false,
 					],
 				]
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/clear.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/clear.php
index bc87124bed..9469b0bb90 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/clear.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/clear.php
@@ -29,6 +29,7 @@ public function set_up() {
 	 */
 	public function testShouldReturnAsExpected( $config, $expected ) {
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 		$this->filesystem->shouldReceive('exists')->with($expected['path'])->andReturn($config['exists']);
 		$this->configureDirDelete($config, $expected);
 		$this->assertSame($expected['output'], $this->filesystemcache->clear());
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/delete.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/delete.php
index e4312c637e..363d58a9d1 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/delete.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/delete.php
@@ -29,6 +29,7 @@ public function set_up() {
 	 */
 	public function testShouldReturnAsExpected( $config, $expected ) {
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 		Functions\expect('get_rocket_parse_url')->with($config['key'])->andReturn($config['parsed_url']);
 		Functions\when('home_url')->justReturn($config['home_url']);
 		$this->filesystem->shouldReceive('exists')->with($expected['path'])->andReturn($config['exists']);
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php
index 457b99df4e..8c72ddae3e 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/deleteMultiple.php
@@ -29,6 +29,7 @@ public function set_up() {
 	 */
 	public function testShouldReturnAsExpected( $config, $expected ) {
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 		Functions\when('home_url')->justReturn($config['home_url']);
 		Functions\when('get_rocket_parse_url')->alias(function ($url) use ($config) {
 			if(! key_exists($url, $config['parsed_url'])) {
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php
index 404627f54b..e71bad742c 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/generateUrl.php
@@ -56,6 +56,7 @@ public function testShouldReturnAsExpected( $config, $expected )
 		});
 
 		Functions\when('home_url')->justReturn($config['home_url']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 
 		Functions\when('rocket_get_constant')->alias(function ($name, $default = false) use ($config) {
 			if('WP_CONTENT_URL' === $name) {
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/get.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/get.php
index 883195cff6..974e6f2854 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/get.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/get.php
@@ -31,6 +31,7 @@ public function testShouldReturnAsExpected( $config, $expected ) {
 		Functions\expect('get_rocket_parse_url')->with($expected['url'])->andReturn($config['parsed_url']);
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
 		Functions\when('home_url')->justReturn($config['home_url']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 
 		$this->filesystem->shouldReceive('exists')->with($expected['path'])->andReturn($config['exists']);
 
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php
index cf63418aa9..38146b76d9 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getMultiple.php
@@ -35,6 +35,7 @@ public function testShouldReturnAsExpected( $config, $expected ) {
 			return $config['parsed_url'][$url];
 		});
 		Functions\when('home_url')->justReturn($config['home_url']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
 
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php
index ae8ceffb11..8eb6e80688 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/getRootPath.php
@@ -43,6 +43,7 @@ public function set_up() {
     public function testShouldReturnAsExpected( $config, $expected )
     {
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
         $this->assertSame($expected, $this->filesystemcache->get_root_path());
     }
 }
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/has.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/has.php
index 48c9473073..757085e4d6 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/has.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/has.php
@@ -31,6 +31,7 @@ public function testShouldReturnAsExpected( $config, $expected ) {
 		Functions\expect('get_rocket_parse_url')->with($expected['url'])->andReturn($config['parsed_url']);
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
 		Functions\when('home_url')->justReturn($config['home_url']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 
 		$this->filesystem->shouldReceive('exists')->with($expected['path'])->andReturn($config['exists']);
 
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php
index a948f18770..43f0c0c2c7 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/isAccessible.php
@@ -30,6 +30,7 @@ public function set_up() {
 	 */
 	public function testShouldReturnAsExpected( $config, $expected ) {
 	Functions\when('rocket_get_constant')->justReturn($config['root']);
+	Functions\when('get_current_blog_id')->justReturn( 1 );
 
 	$this->filesystem->shouldReceive('exists')->with($expected['path'])->andReturn($config['exists']);
 	if( ! $config['exists']) {
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/set.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/set.php
index 34fa4a0c1a..64d9410089 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/set.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/set.php
@@ -34,6 +34,7 @@ public function testShouldReturnAsExpected( $config, $expected ) {
 	Functions\when('rocket_get_constant')->justReturn($config['root']);
 	Functions\expect('rocket_mkdir_p')->with( dirname($expected['path']), $this->filesystem );
 	Functions\when('home_url')->justReturn($config['home_url']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 
 	$this->filesystem->shouldReceive('put_contents')->with($expected['path'], $expected['content'], $config['rights'])->andReturn($config['saved']);
 
diff --git a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php
index 2d7eb2da32..0c4546a854 100644
--- a/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php
+++ b/tests/Unit/inc/Engine/Common/Cache/FilesystemCache/setMultiple.php
@@ -40,6 +40,7 @@ public function testShouldReturnAsExpected( $config, $expected ) {
 		Functions\when('rocket_mkdir_p')->justReturn();
 
 		Functions\when('rocket_get_constant')->justReturn($config['root']);
+		Functions\when('get_current_blog_id')->justReturn( 1 );
 
 		foreach ($config['saved'] as $path => $saved) {
 			$this->filesystem->shouldReceive('put_contents')->with($path, $saved['content'], $config['rights'])->andReturn($saved['output']);