diff --git a/libraries/persistent_store/src/storage.rs b/libraries/persistent_store/src/storage.rs
index adc3c53..696c258 100644
--- a/libraries/persistent_store/src/storage.rs
+++ b/libraries/persistent_store/src/storage.rs
@@ -62,6 +62,9 @@ pub trait Storage {
     /// Reads a byte slice from the storage.
     ///
     /// The `index` must designate `length` bytes in the storage.
+    ///
+    /// Note that we use `Cow` just because it derefs to `[u8]`. We don't really need the fact that
+    /// one can convert it to a `Vec`. In particular we don't do it in the store implementation.
     fn read_slice(&self, index: StorageIndex, length: usize) -> StorageResult<Cow<[u8]>>;
 
     /// Writes a word slice to the storage.