diff --git a/src/mp4.cc b/src/mp4.cc
index ec5fc90..91bbfff 100644
--- a/src/mp4.cc
+++ b/src/mp4.cc
@@ -548,6 +548,7 @@ class Mp4File : public VirtualFile {
       etag_digest->Update(segment->recording.sample_file_sha1);
     }
     etag_ = StrCat("\"", ToHex(etag_digest->Finalize()), "\"");
+    VLOG(1) << "Constructed .mp4 has " << slices_.size() << " slices.";
   }
 
   time_t last_modified() const final { return last_modified_; }