Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 21255)
+++ wp-includes/ms-functions.php	(working copy)
@@ -1459,34 +1459,6 @@
 }
 
 /**
- * Check whether a blog has used its allotted upload space.
- *
- * @since MU
- * @uses get_dirsize()
- *
- * @param bool $echo Optional. If $echo is set and the quota is exceeded, a warning message is echoed. Default is true.
- * @return int
- */
-function upload_is_user_over_quota( $echo = true ) {
-	if ( get_site_option( 'upload_space_check_disabled' ) )
-		return false;
-
-	$spaceAllowed = get_space_allowed();
-	if ( empty( $spaceAllowed ) || !is_numeric( $spaceAllowed ) )
-		$spaceAllowed = 10;	// Default space allowed is 10 MB
-
-	$size = get_dirsize( BLOGUPLOADDIR ) / 1024 / 1024;
-
-	if ( ($spaceAllowed-$size) < 0 ) {
-		if ( $echo )
-			_e( 'Sorry, you have used your space allocation. Please delete some files to upload more files.' ); // No space left
-		return true;
-	} else {
-		return false;
-	}
-}
-
-/**
  * Check an array of MIME types against a whitelist.
  *
  * WordPress ships with a set of allowed upload filetypes,
@@ -1541,29 +1513,6 @@
 }
 
 /**
- * Get the remaining upload space for this blog.
- *
- * @since MU
- * @uses upload_is_user_over_quota()
- * @uses get_space_allowed()
- * @uses get_dirsize()
- *
- * @param int $size
- * @return int
- */
-function fix_import_form_size( $size ) {
-	if ( upload_is_user_over_quota( false ) == true )
-		return 0;
-
-	$spaceAllowed = 1024 * 1024 * get_space_allowed();
-	$dirsize = get_dirsize( BLOGUPLOADDIR );
-	if ( $size > $spaceAllowed - $dirsize )
-		return $spaceAllowed - $dirsize; // remaining space
-	else
-		return $size; // default
-}
-
-/**
  * Maintains a canonical list of terms by syncing terms created for each blog with the global terms table.
  *
  * @since 3.0.0
Index: wp-admin/includes/dashboard.php
===================================================================
--- wp-admin/includes/dashboard.php	(revision 21255)
+++ wp-admin/includes/dashboard.php	(working copy)
@@ -1097,7 +1097,7 @@
 		return true;
 
 	$quota = get_space_allowed();
-	$used = get_dirsize( BLOGUPLOADDIR ) / 1024 / 1024;
+	$used = get_space_used();
 
 	if ( $used > $quota )
 		$percentused = '100';
Index: wp-admin/includes/ms.php
===================================================================
--- wp-admin/includes/ms.php	(revision 21255)
+++ wp-admin/includes/ms.php	(working copy)
@@ -25,9 +25,8 @@
 	if ( defined( 'WP_IMPORTING' ) )
 		return $file;
 
-	$space_allowed = 1048576 * get_space_allowed();
-	$space_used = get_dirsize( BLOGUPLOADDIR );
-	$space_left = $space_allowed - $space_used;
+	$space_left = get_upload_space_available();
+
 	$file_size = filesize( $file['tmp_name'] );
 	if ( $space_left < $file_size )
 		$file['error'] = sprintf( __( 'Not enough space to upload. %1$s KB needed.' ), number_format( ($file_size - $space_left) /1024 ) );
@@ -310,28 +309,48 @@
 	if ( get_site_option( 'upload_space_check_disabled' ) )
 		return $space_allowed;
 
-	$dir_name = trailingslashit( BLOGUPLOADDIR );
-	if ( !( is_dir( $dir_name) && is_readable( $dir_name ) ) )
-		return $space_allowed;
+	$space_used = get_space_used() * 1024 * 1024;
 
-  	$dir = dir( $dir_name );
-   	$size = 0;
+	if ( ( $space_allowed - $space_used ) <= 0 )
+		return 0;
 
-	while ( $file = $dir->read() ) {
-		if ( $file != '.' && $file != '..' ) {
-			if ( is_dir( $dir_name . $file) ) {
-				$size += get_dirsize( $dir_name . $file );
-			} else {
-				$size += filesize( $dir_name . $file );
-			}
-		}
+	return $space_allowed - $space_used;
+}
+
+/**
+ * Check whether a blog has used its allotted upload space.
+ *
+ * @since MU
+ *
+ * @param bool $echo Optional. If $echo is set and the quota is exceeded, a warning message is echoed. Default is true.
+ * @return int
+ */
+function upload_is_user_over_quota( $echo = true ) {
+	if ( get_site_option( 'upload_space_check_disabled' ) )
+		return false;
+
+	$space_allowed = get_space_allowed();
+	if ( empty( $space_allowed ) || !is_numeric( $space_allowed ) )
+		$space_allowed = 10;// Default space allowed is 10 MB
+
+	$space_used = get_space_used();
+
+	if ( ($space_allowed - $space_used ) < 0 ) {
+		if ( $echo )
+			_e( 'Sorry, you have used your space allocation. Please delete some files to upload more files.' );
+		return true;
+	} else {
+		return false;
 	}
-	$dir->close();
+}
 
-	if ( ( $space_allowed - $size ) <= 0 )
-		return 0;
+function get_space_used() {
+	// Allow for an alternative way of tracking storage space used
+	$space_used = apply_filters( 'pre_get_space_used', false );
+	if ( false === $space_used )
+		$space_used = get_dirsize( BLOGUPLOADDIR );
 
-	return $space_allowed - $size;
+	return $space_used;
 }
 
 /**
@@ -352,24 +371,44 @@
 }
 
 function display_space_usage() {
-	$space = get_space_allowed();
-	$used = get_dirsize( BLOGUPLOADDIR ) / 1024 / 1024;
+	$space_allowed = get_space_allowed();
+	$space_used = get_space_used();
 
-	$percentused = ( $used / $space ) * 100;
+	$percent_used = ( $space_used / $space_allowed ) * 100;
 
-	if ( $space > 1000 ) {
-		$space = number_format( $space / 1024 );
+	if ( $space_allowed > 1000 ) {
+		$space = number_format( $space_allowed / 1024 );
 		/* translators: Gigabytes */
 		$space .= __( 'GB' );
 	} else {
+		$space = number_format( $space_allowed );
 		/* translators: Megabytes */
 		$space .= __( 'MB' );
 	}
 	?>
-	<strong><?php printf( __( 'Used: %1s%% of %2s' ), number_format( $percentused ), $space ); ?></strong>
+	<strong><?php printf( __( 'Used: %1s%% of %2s' ), number_format( $percent_used ), $space ); ?></strong>
 	<?php
 }
 
+/**
+ * Get the remaining upload space for this blog.
+ *
+ * @since MU
+ * @uses upload_is_user_over_quota()
+ * @uses get_space_allowed()
+ * @uses get_upload_space_available()
+ *
+ * @param int $size Current max size in bytes
+ * @return int Max size in bytes
+ */
+function fix_import_form_size( $size ) {
+	if ( upload_is_user_over_quota( false ) == true )
+		return 0;
+
+	$available = get_upload_space_available();
+	return min( $size, $available);
+}
+
 // Edit blog upload space setting on Edit Blog page
 function upload_space_setting( $id ) {
 	$quota = get_blog_option( $id, 'blog_upload_space' );