diff --git a/lib/blob/blobstore.h b/lib/blob/blobstore.h
index 83f810634..8d7e8f3d4 100644
--- a/lib/blob/blobstore.h
+++ b/lib/blob/blobstore.h
@@ -245,11 +245,18 @@ struct spdk_bs_md_mask {
 };
 
 #define SPDK_MD_DESCRIPTOR_TYPE_PADDING 0
-#define SPDK_MD_DESCRIPTOR_TYPE_EXTENT_RLE 1
 #define SPDK_MD_DESCRIPTOR_TYPE_XATTR 2
 #define SPDK_MD_DESCRIPTOR_TYPE_FLAGS 3
 #define SPDK_MD_DESCRIPTOR_TYPE_XATTR_INTERNAL 4
 
+/* Following descriptors define cluster layout in a blob. */
+
+/* EXTENT_RLE descriptor holds an array of LBA that points to
+ * beginning of allocated clusters. The array is run-length encoded,
+ * with 0's being unallocated clusters. It is part of serialized
+ * metadata chain for a blob. */
+#define SPDK_MD_DESCRIPTOR_TYPE_EXTENT_RLE 1
+
 struct spdk_blob_md_descriptor_xattr {
 	uint8_t		type;
 	uint32_t	length;