diff --git a/docs/formatbackup.rst b/docs/formatbackup.rst
index 1b9e6ebb22..c378ad9d9a 100644
--- a/docs/formatbackup.rst
+++ b/docs/formatbackup.rst
@@ -95,6 +95,10 @@ were supplied). The following child elements and attributes are supported:
          Similar to a disk declaration for a domain, the choice of type controls
          what additional sub-elements are needed to describe the destination.
 
+      ``index``
+         Output only. The value can be used to refer to the scratch or output
+         file of the backup in APIs such as ``virDomainSetBlockThreshold``.
+
       ``target``
          Valid only for push mode backups, this is the primary sub-element that
          describes the file name of the backup destination, similar to the
diff --git a/src/conf/backup_conf.c b/src/conf/backup_conf.c
index 11d419ce2b..90ffcc51d1 100644
--- a/src/conf/backup_conf.c
+++ b/src/conf/backup_conf.c
@@ -105,6 +105,7 @@ virDomainBackupDiskDefParseXML(xmlNodePtr node,
     VIR_XPATH_NODE_AUTORESTORE(ctxt)
     g_autofree char *type = NULL;
     g_autofree char *format = NULL;
+    g_autofree char *idx = NULL;
     g_autofree char *backup = NULL;
     g_autofree char *state = NULL;
     g_autofree char *backupmode = NULL;
@@ -171,8 +172,10 @@ virDomainBackupDiskDefParseXML(xmlNodePtr node,
 
     type = virXMLPropString(node, "type");
     format = virXPathString("string(./driver/@type)", ctxt);
+    if (internal)
+        idx = virXMLPropString(node, "index");
 
-    if (!(def->store = virDomainStorageSourceParseBase(type, format, NULL)))
+    if (!(def->store = virDomainStorageSourceParseBase(type, format, idx)))
           return -1;
 
     if (def->store->type != VIR_STORAGE_TYPE_FILE &&
@@ -386,6 +389,9 @@ virDomainBackupDiskDefFormat(virBufferPtr buf,
         virBufferEscapeString(&attrBuf, " exportname='%s'", disk->exportname);
         virBufferEscapeString(&attrBuf, " exportbitmap='%s'", disk->exportbitmap);
 
+        if (disk->store->id != 0)
+            virBufferAsprintf(&attrBuf, " index='%u'", disk->store->id);
+
         if (disk->store->format > 0)
             virBufferEscapeString(&childBuf, "<driver type='%s'/>\n",
                                   virStorageFileFormatTypeToString(disk->store->format));
diff --git a/tests/qemustatusxml2xmldata/backup-pull-in.xml b/tests/qemustatusxml2xmldata/backup-pull-in.xml
index faaed67e38..bb322f94d3 100644
--- a/tests/qemustatusxml2xmldata/backup-pull-in.xml
+++ b/tests/qemustatusxml2xmldata/backup-pull-in.xml
@@ -256,7 +256,7 @@
       <incremental>12345</incremental>
       <server transport='tcp' tls='yes' name='localhost' port='10809'/>
       <disks>
-        <disk name='vda' backup='yes' state='running' type='file'>
+        <disk name='vda' backup='yes' state='running' type='file' index='123'>
           <scratch file='/path/to/file/'/>
         </disk>
       </disks>