diff --git a/src/garage/cli/layout.rs b/src/garage/cli/layout.rs
index 0d9e4fa4..e76f7737 100644
--- a/src/garage/cli/layout.rs
+++ b/src/garage/cli/layout.rs
@@ -43,7 +43,11 @@ pub async fn cmd_assign_role(
 		resp => return Err(Error::Message(format!("Invalid RPC response: {:?}", resp))),
 	};
 
-	let added_node = find_matching_node(status.iter().map(|adv| adv.id), &args.node_id)?;
+	let added_nodes = args
+		.node_ids
+		.iter()
+		.map(|node_id| find_matching_node(status.iter().map(|adv| adv.id), node_id))
+		.collect::<Result<Vec<_>, _>>()?;
 
 	let mut layout = fetch_layout(rpc_cli, rpc_host).await?;
 
@@ -75,46 +79,51 @@ pub async fn cmd_assign_role(
 		return Err(Error::Message("Invalid capacity value: 0".into()));
 	}
 
-	let new_entry = match roles.get(&added_node) {
-		Some(NodeRoleV(Some(old))) => {
-			let capacity = match args.capacity {
-				Some(c) => Some(c),
-				None if args.gateway => None,
-				None => old.capacity,
-			};
-			let tags = if args.tags.is_empty() {
-				old.tags.clone()
-			} else {
-				args.tags
-			};
-			NodeRole {
-				zone: args.zone.unwrap_or_else(|| old.zone.to_string()),
-				capacity,
-				tags,
+	for added_node in added_nodes {
+		let new_entry = match roles.get(&added_node) {
+			Some(NodeRoleV(Some(old))) => {
+				let capacity = match args.capacity {
+					Some(c) => Some(c),
+					None if args.gateway => None,
+					None => old.capacity,
+				};
+				let tags = if args.tags.is_empty() {
+					old.tags.clone()
+				} else {
+					args.tags.clone()
+				};
+				NodeRole {
+					zone: args.zone.clone().unwrap_or_else(|| old.zone.to_string()),
+					capacity,
+					tags,
+				}
 			}
-		}
-		_ => {
-			let capacity = match args.capacity {
-				Some(c) => Some(c),
-				None if args.gateway => None,
-				None => return Err(Error::Message(
-						"Please specify a capacity with the -c flag, or set node explicitly as gateway with -g".into())),
-			};
-			NodeRole {
-				zone: args.zone.ok_or("Please specifiy a zone with the -z flag")?,
-				capacity,
-				tags: args.tags,
+			_ => {
+				let capacity = match args.capacity {
+					Some(c) => Some(c),
+					None if args.gateway => None,
+					None => return Err(Error::Message(
+							"Please specify a capacity with the -c flag, or set node explicitly as gateway with -g".into())),
+				};
+				NodeRole {
+					zone: args
+						.zone
+						.clone()
+						.ok_or("Please specifiy a zone with the -z flag")?,
+					capacity,
+					tags: args.tags.clone(),
+				}
 			}
-		}
-	};
+		};
 
-	layout
-		.staging
-		.merge(&roles.update_mutator(added_node, NodeRoleV(Some(new_entry))));
+		layout
+			.staging
+			.merge(&roles.update_mutator(added_node, NodeRoleV(Some(new_entry))));
+	}
 
 	send_layout(rpc_cli, rpc_host, layout).await?;
 
-	println!("Role change is staged but not yet commited.");
+	println!("Role changes are staged but not yet commited.");
 	println!("Use `garage layout show` to view staged role changes,");
 	println!("and `garage layout apply` to enact staged changes.");
 	Ok(())
diff --git a/src/garage/cli/structs.rs b/src/garage/cli/structs.rs
index 30cbb2da..a0c49aeb 100644
--- a/src/garage/cli/structs.rs
+++ b/src/garage/cli/structs.rs
@@ -92,8 +92,9 @@ pub enum LayoutOperation {
 
 #[derive(StructOpt, Debug)]
 pub struct AssignRoleOpt {
-	/// Node to which to assign role (prefix of hexadecimal node id)
-	pub(crate) node_id: String,
+	/// Node(s) to which to assign role (prefix of hexadecimal node id)
+	#[structopt(required = true)]
+	pub(crate) node_ids: Vec<String>,
 
 	/// Location (zone or datacenter) of the node
 	#[structopt(short = "z", long = "zone")]
diff --git a/src/rpc/layout.rs b/src/rpc/layout.rs
index 895dbf1c..b9c02c21 100644
--- a/src/rpc/layout.rs
+++ b/src/rpc/layout.rs
@@ -172,30 +172,43 @@ impl ClusterLayout {
 		println!("Calculating updated partition assignation, this may take some time...");
 		println!();
 
+		// Get old partition assignation
 		let old_partitions = self.parse_assignation_data();
 
-		let mut partitions = old_partitions.clone();
-		for part in partitions.iter_mut() {
-			part.nodes
-				.retain(|(_, info)| info.map(|x| x.capacity.is_some()).unwrap_or(false));
-		}
+		// Start new partition assignation with nodes from old assignation where it is relevant
+		let mut partitions = old_partitions
+			.iter()
+			.map(|old_part| {
+				let mut new_part = PartitionAss::new();
+				for node in old_part.nodes.iter() {
+					if let Some(role) = node.1 {
+						if role.capacity.is_some() {
+							new_part.add(None, n_zones, node.0, role);
+						}
+					}
+				}
+				new_part
+			})
+			.collect::<Vec<_>>();
 
-		// When nodes are removed, or when bootstraping an assignation from
-		// scratch for a new cluster, the old partitions will have holes (or be empty).
-		// Here we add more nodes to make a complete (sub-optimal) assignation,
+		// In various cases, not enough nodes will have been added for all partitions
+		// in the step above (e.g. due to node removals, or new zones being added).
+		// Here we add more nodes to make a complete (but sub-optimal) assignation,
 		// using an initial partition assignation that is calculated using the multi-dc maglev trick
 		match self.initial_partition_assignation() {
 			Some(initial_partitions) => {
 				for (part, ipart) in partitions.iter_mut().zip(initial_partitions.iter()) {
 					for (id, info) in ipart.nodes.iter() {
 						if part.nodes.len() < self.replication_factor {
-							part.add(part.nodes.len() + 1, n_zones, id, info.unwrap());
+							part.add(None, n_zones, id, info.unwrap());
 						}
 					}
 					assert!(part.nodes.len() == self.replication_factor);
 				}
 			}
 			None => {
+				// Not enough nodes in cluster to build a correct assignation.
+				// Signal it by returning an error.
 				return false;
 			}
 		}
@@ -232,8 +245,13 @@ impl ClusterLayout {
 			let mut option = None;
 			for (i, part) in partitions.iter_mut().enumerate() {
 				for (irm, (idrm, _)) in part.nodes.iter().enumerate() {
-					let suprm = partitions_per_node.get(*idrm).cloned().unwrap_or(0) as i32
-						- target_partitions_per_node.get(*idrm).cloned().unwrap_or(0) as i32;
+					let errratio = |node, parts| {
+						let tgt = *target_partitions_per_node.get(node).unwrap() as f32;
+						(parts - tgt) / tgt
+					};
+					let square = |x| x * x;
+
+					let partsrm = partitions_per_node.get(*idrm).cloned().unwrap_or(0) as f32;
 
 					for (idadd, infoadd) in configured_nodes.iter() {
 						// skip replacing a node by itself
@@ -242,14 +260,12 @@ impl ClusterLayout {
 							continue;
 						}
 
-						let supadd = partitions_per_node.get(*idadd).cloned().unwrap_or(0) as i32
-							- target_partitions_per_node.get(*idadd).cloned().unwrap_or(0) as i32;
-
 						// We want to try replacing node idrm by node idadd
 						// if that brings us close to our goal.
-						let square = |i: i32| i * i;
-						let oldcost = square(suprm) + square(supadd);
-						let newcost = square(suprm - 1) + square(supadd + 1);
+						let partsadd = partitions_per_node.get(*idadd).cloned().unwrap_or(0) as f32;
+						let oldcost = square(errratio(*idrm, partsrm) - errratio(*idadd, partsadd));
+						let newcost =
+							square(errratio(*idrm, partsrm - 1.) - errratio(*idadd, partsadd + 1.));
 						if newcost >= oldcost {
 							// not closer to our goal
 							continue;
@@ -259,7 +275,7 @@ impl ClusterLayout {
 						let mut newpart = part.clone();
 
 						newpart.nodes.remove(irm);
-						if !newpart.add(newpart.nodes.len() + 1, n_zones, idadd, infoadd) {
+						if !newpart.add(None, n_zones, idadd, infoadd) {
 							continue;
 						}
 						assert!(newpart.nodes.len() == self.replication_factor);
@@ -302,7 +318,9 @@ impl ClusterLayout {
 		// Show statistics
 		println!("New number of partitions per node:");
 		for (node, npart) in partitions_per_node.iter() {
-			println!("{:?}\t{}", node, npart);
+			let tgt = *target_partitions_per_node.get(node).unwrap();
+			let pct = 100f32 * (*npart as f32) / (tgt as f32);
+			println!("{:?}\t{}\t({}% of {})", node, npart, pct as i32, tgt);
 		}
 		println!();
 
@@ -394,7 +412,7 @@ impl ClusterLayout {
 							continue;
 						}
 						for (pos2, &qv) in q.iter().enumerate().skip(*pos) {
-							if partitions[qv].add(rep + 1, n_zones, node_id, node_info) {
+							if partitions[qv].add(Some(rep + 1), n_zones, node_id, node_info) {
 								remaining -= 1;
 								*pos = pos2 + 1;
 								break;
@@ -551,15 +569,26 @@ impl<'a> PartitionAss<'a> {
 		}
 	}
 
+	// add is a key function in creating a PartitionAss, i.e. the list of nodes
+	// to which a partition is assigned. It tries to add a certain node id to the
+	// assignation, but checks that doing so is compatible with the NECESSARY
+	// condition that the partition assignation must be dispersed over different
+	// zones (datacenters) if enough zones exist. This is why it takes a n_zones
+	// parameter, which is the total number of zones that have existing nodes:
+	// if nodes in the assignation already cover all n_zones zones, then any node
+	// that is not yet in the assignation can be added. Otherwise, only nodes
+	// that are in a new zone can be added.
 	fn add(
 		&mut self,
-		target_len: usize,
+		target_len: Option<usize>,
 		n_zones: usize,
 		node: &'a Uuid,
 		role: &'a NodeRole,
 	) -> bool {
-		if self.nodes.len() != target_len - 1 {
-			return false;
+		if let Some(tl) = target_len {
+			if self.nodes.len() != tl - 1 {
+				return false;
+			}
 		}
 
 		let p_zns = self