diff --git a/util/src/hashdb.rs b/util/src/hashdb.rs
index 35606e1ba..51b9d445e 100644
--- a/util/src/hashdb.rs
+++ b/util/src/hashdb.rs
@@ -114,6 +114,11 @@ pub trait HashDB: AsHashDB {
 	fn get_aux(&self, _hash: &[u8]) -> Option<Vec<u8>> {
 		unimplemented!();
 	}
+
+	/// Removes auxiliary data from hashdb.
+	fn remove_aux(&mut self, _hash: &[u8]) {
+		unimplemented!();
+	}
 }
 
 /// Upcast trait.
diff --git a/util/src/journaldb/archivedb.rs b/util/src/journaldb/archivedb.rs
index 40b322802..3713fc79e 100644
--- a/util/src/journaldb/archivedb.rs
+++ b/util/src/journaldb/archivedb.rs
@@ -26,8 +26,13 @@ use kvdb::{Database, DBTransaction, DatabaseConfig};
 #[cfg(test)]
 use std::env;
 
+/// Suffix appended to auxiliary keys to distinguish them from normal keys.
+/// Would be nich to use rocksdb columns for this eventually.
 const AUX_FLAG: u8 = 255;
 
+/// Database version.
+const DB_VERSION : u32 = 0x103;
+
 /// Implementation of the `HashDB` trait for a disk-backed database with a memory overlay
 /// and latent-removal semantics.
 ///
@@ -41,8 +46,6 @@ pub struct ArchiveDB {
 	latest_era: Option<u64>,
 }
 
-const DB_VERSION : u32 = 0x103;
-
 impl ArchiveDB {
 	/// Create a new instance from file
 	pub fn new(path: &str, cache_size: Option<usize>) -> ArchiveDB {
@@ -122,9 +125,11 @@ impl HashDB for ArchiveDB {
 	fn insert(&mut self, value: &[u8]) -> H256 {
 		self.overlay.insert(value)
 	}
+
 	fn emplace(&mut self, key: H256, value: Bytes) {
 		self.overlay.emplace(key, value);
 	}
+
 	fn remove(&mut self, key: &H256) {
 		self.overlay.remove(key);
 	}
@@ -145,6 +150,10 @@ impl HashDB for ArchiveDB {
 			.expect("Low-level database error. Some issue with your hard disk?")
 			.map(|v| v.to_vec())
 	}
+
+	fn remove_aux(&mut self, hash: &[u8]) {
+		self.overlay.remove_aux(hash);
+	}
 }
 
 impl JournalDB for ArchiveDB {
diff --git a/util/src/memorydb.rs b/util/src/memorydb.rs
index f62c6b9b2..f63dfd992 100644
--- a/util/src/memorydb.rs
+++ b/util/src/memorydb.rs
@@ -136,16 +136,12 @@ impl MemoryDB {
 
 	/// Return the internal map of hashes to data, clearing the current state.
 	pub fn drain(&mut self) -> HashMap<H256, (Bytes, i32)> {
-		let mut data = HashMap::new();
-		mem::swap(&mut self.data, &mut data);
-		data
+		mem::replace(&mut self.data, HashMap::new())
 	}
 
 	/// Return the internal map of auxiliary data, clearing the current state.
 	pub fn drain_aux(&mut self) -> HashMap<Bytes, Bytes> {
-		let mut aux = HashMap::new();
-		mem::swap(&mut self.aux, &mut aux);
-		aux
+		mem::replace(&mut self.aux, HashMap::new())
 	}
 
 	/// Denote than an existing value has the given key. Used when a key gets removed without
@@ -250,6 +246,10 @@ impl HashDB for MemoryDB {
 	fn get_aux(&self, hash: &[u8]) -> Option<Vec<u8>> {
 		self.aux.get(hash).cloned()
 	}
+
+	fn remove_aux(&mut self, hash: &[u8]) {
+		self.aux.remove(hash);
+	}
 }
 
 #[test]