diff --git a/MoonTools.ECS.csproj b/MoonTools.ECS.csproj
index 8867478..e7a1568 100644
--- a/MoonTools.ECS.csproj
+++ b/MoonTools.ECS.csproj
@@ -4,6 +4,7 @@
     <TargetFramework>net6.0</TargetFramework>
     <Nullable>enable</Nullable>
     <Platforms>x64</Platforms>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
 </Project>
diff --git a/src/ComponentDepot.cs b/src/ComponentDepot.cs
index bd43661..4c063ea 100644
--- a/src/ComponentDepot.cs
+++ b/src/ComponentDepot.cs
@@ -11,13 +11,13 @@ namespace MoonTools.ECS
 
 		private Dictionary<Type, HashSet<FilterSignature>> typeToFilterSignatures = new Dictionary<Type, HashSet<FilterSignature>>();
 
-		private Dictionary<int, HashSet<Type>> entityComponentMap = new Dictionary<int, HashSet<Type>>();
-
 #if DEBUG
 		private Dictionary<Type, Filter> singleComponentFilters = new Dictionary<Type, Filter>();
 #endif
 
-		internal void Register<TComponent>() where TComponent : struct
+		private HashSet<Type> TypesWithDisabledSerialization = new HashSet<Type>();
+
+		internal void Register<TComponent>() where TComponent : unmanaged
 		{
 			if (!storages.ContainsKey(typeof(TComponent)))
 			{
@@ -33,19 +33,19 @@ namespace MoonTools.ECS
 			return storages[type];
 		}
 
-		private ComponentStorage<TComponent> Lookup<TComponent>() where TComponent : struct
+		private ComponentStorage<TComponent> Lookup<TComponent>() where TComponent : unmanaged
 		{
 			// TODO: is it possible to optimize this?
 			Register<TComponent>();
 			return (ComponentStorage<TComponent>) storages[typeof(TComponent)];
 		}
 
-		public bool Some<TComponent>() where TComponent : struct
+		public bool Some<TComponent>() where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().Any();
 		}
 
-		public bool Has<TComponent>(int entityID) where TComponent : struct
+		public bool Has<TComponent>(int entityID) where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().Has(entityID);
 		}
@@ -55,29 +55,22 @@ namespace MoonTools.ECS
 			return Lookup(type).Has(entityID);
 		}
 
-		public ref readonly TComponent Get<TComponent>(int entityID) where TComponent : struct
+		public ref readonly TComponent Get<TComponent>(int entityID) where TComponent : unmanaged
 		{
 			return ref Lookup<TComponent>().Get(entityID);
 		}
 
-		public ref readonly TComponent Get<TComponent>() where TComponent : struct
+		public ref readonly TComponent Get<TComponent>() where TComponent : unmanaged
 		{
 			return ref Lookup<TComponent>().Get();
 		}
 
-		public void Set<TComponent>(int entityID, in TComponent component) where TComponent : struct
+		public void Set<TComponent>(int entityID, in TComponent component) where TComponent : unmanaged
 		{
-			Lookup<TComponent>().Set(entityID, component);
-
-			if (!entityComponentMap.ContainsKey(entityID))
-			{
-				entityComponentMap.Add(entityID, new HashSet<Type>());
-			}
-
-			var notFound = entityComponentMap[entityID].Add(typeof(TComponent));
+			var existed = Lookup<TComponent>().Set(entityID, component);
 
 			// update filters
-			if (notFound)
+			if (!existed)
 			{
 				if (typeToFilterSignatures.TryGetValue(typeof(TComponent), out var filterSignatures))
 				{
@@ -89,24 +82,22 @@ namespace MoonTools.ECS
 			}
 		}
 
-		public Entity GetSingletonEntity<TComponent>() where TComponent : struct
+		public Entity GetSingletonEntity<TComponent>() where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().FirstEntity();
 		}
 
-		public ReadOnlySpan<TComponent> ReadComponents<TComponent>() where TComponent : struct
+		public ReadOnlySpan<TComponent> ReadComponents<TComponent>() where TComponent : unmanaged
 		{
 			return Lookup<TComponent>().AllComponents();
 		}
 
 		private void Remove(Type type, int entityID)
 		{
-			Lookup(type).Remove(entityID);
-
-			var found = entityComponentMap[entityID].Remove(type);
+			var existed = Lookup(type).Remove(entityID);
 
 			// update filters
-			if (found)
+			if (existed)
 			{
 				if (typeToFilterSignatures.TryGetValue(type, out var filterSignatures))
 				{
@@ -118,14 +109,12 @@ namespace MoonTools.ECS
 			}
 		}
 
-		public void Remove<TComponent>(int entityID) where TComponent : struct
+		public void Remove<TComponent>(int entityID) where TComponent : unmanaged
 		{
-			Lookup<TComponent>().Remove(entityID);
-
-			var found = entityComponentMap[entityID].Remove(typeof(TComponent));
+			var existed = Lookup<TComponent>().Remove(entityID);
 
 			// update filters
-			if (found)
+			if (existed)
 			{
 				if (typeToFilterSignatures.TryGetValue(typeof(TComponent), out var filterSignatures))
 				{
@@ -137,16 +126,12 @@ namespace MoonTools.ECS
 			}
 		}
 
+		// TODO: is there some way to optimize this without complicating serialization?
 		public void OnEntityDestroy(int entityID)
 		{
-			if (entityComponentMap.ContainsKey(entityID))
+			foreach (var type in storages.Keys)
 			{
-				foreach (var type in entityComponentMap[entityID])
-				{
-					Remove(type, entityID);
-				}
-
-				entityComponentMap.Remove(entityID);
+				Remove(type, entityID);
 			}
 		}
 
@@ -231,6 +216,11 @@ namespace MoonTools.ECS
 			filterSignatureToEntityIDs[filterSignature].Add(entityID);
 		}
 
+		public void DisableSerialization<TComponent>() where TComponent : unmanaged
+		{
+			TypesWithDisabledSerialization.Add(typeof(TComponent));
+		}
+
 		public ComponentDepotState CreateState()
 		{
 			return new ComponentDepotState();
@@ -238,12 +228,30 @@ namespace MoonTools.ECS
 
 		public void Serialize(ComponentDepotState state)
 		{
+			// FIXME: this is creating garbage
 			state.StorageStates.Clear();
 			foreach (var (type, storage) in storages)
 			{
-				var storageState = storage.CreateState();
-				storage.Serialize(storageState);
-				state.StorageStates.Add(type, storageState);
+				// FIXME: we could cache this
+				if (!TypesWithDisabledSerialization.Contains(type))
+				{
+					var storageState = storage.CreateState();
+					storage.Serialize(storageState);
+					state.StorageStates.Add(type, storageState);
+				}
+			}
+
+			// FIXME: this is creating garbage
+			state.FilterStates.Clear();
+			foreach (var (signature, set) in filterSignatureToEntityIDs)
+			{
+				// FIXME: we could cache this
+				if (!signature.Included.Overlaps(TypesWithDisabledSerialization) && !signature.Excluded.Overlaps(TypesWithDisabledSerialization))
+				{
+					var setState = new IndexableSetState<int>(set.Count);
+					set.Save(setState);
+					state.FilterStates[signature] = setState;
+				}
 			}
 		}
 
@@ -253,6 +261,11 @@ namespace MoonTools.ECS
 			{
 				storages[type].Deserialize(storageState);
 			}
+
+			foreach (var (signature, setState) in state.FilterStates)
+			{
+				filterSignatureToEntityIDs[signature].Load(setState);
+			}
 		}
 
 #if DEBUG
diff --git a/src/ComponentDepotState.cs b/src/ComponentDepotState.cs
index 78481d1..b137f24 100644
--- a/src/ComponentDepotState.cs
+++ b/src/ComponentDepotState.cs
@@ -3,8 +3,9 @@ using System.Collections.Generic;
 
 namespace MoonTools.ECS
 {
-    public class ComponentDepotState
-    {
+	public class ComponentDepotState
+	{
 		public Dictionary<Type, ComponentStorageState> StorageStates = new Dictionary<Type, ComponentStorageState>();
+		public Dictionary<FilterSignature, IndexableSetState<int>> FilterStates = new Dictionary<FilterSignature, IndexableSetState<int>>();
 	}
 }
diff --git a/src/ComponentStorage.cs b/src/ComponentStorage.cs
index 3020bb1..e19af89 100644
--- a/src/ComponentStorage.cs
+++ b/src/ComponentStorage.cs
@@ -7,14 +7,14 @@ namespace MoonTools.ECS
 	internal abstract class ComponentStorage
 	{
 		public abstract bool Has(int entityID);
-		public abstract void Remove(int entityID);
+		public abstract bool Remove(int entityID);
 		public abstract object Debug_Get(int entityID);
 		public abstract ComponentStorageState CreateState();
 		public abstract void Serialize(ComponentStorageState state);
 		public abstract void Deserialize(ComponentStorageState state);
 	}
 
-	internal class ComponentStorage<TComponent> : ComponentStorage where TComponent : struct
+	internal class ComponentStorage<TComponent> : ComponentStorage where TComponent : unmanaged
 	{
 		private int nextID;
 		private readonly Dictionary<int, int> entityIDToStorageIndex = new Dictionary<int, int>(16);
@@ -52,8 +52,11 @@ namespace MoonTools.ECS
 			return ref components[0];
 		}
 
-		public void Set(int entityID, in TComponent component)
+		// Returns true if the entity already had this component.
+		public bool Set(int entityID, in TComponent component)
 		{
+			bool result = true;
+
 			if (!entityIDToStorageIndex.ContainsKey(entityID))
 			{
 				var index = nextID;
@@ -67,12 +70,17 @@ namespace MoonTools.ECS
 
 				entityIDToStorageIndex[entityID] = index;
 				entityIDs[index] = entityID;
+
+				result = false;
 			}
 
 			components[entityIDToStorageIndex[entityID]] = component;
+
+			return result;
 		}
 
-		public override void Remove(int entityID)
+		// Returns true if the entity had this component.
+		public override bool Remove(int entityID)
 		{
 			if (entityIDToStorageIndex.ContainsKey(entityID))
 			{
@@ -91,7 +99,11 @@ namespace MoonTools.ECS
 				}
 
 				nextID -= 1;
+
+				return true;
 			}
+
+			return false;
 		}
 
 		public void Clear()
@@ -137,6 +149,8 @@ namespace MoonTools.ECS
 			{
 				serializedComponentStorage.EntityIdToStorageIndex[kvp.Key] = kvp.Value;
 			}
+
+			serializedComponentStorage.Count = nextID;
 		}
 
 		public override void Deserialize(ComponentStorageState serializedComponentStorage)
@@ -149,6 +163,8 @@ namespace MoonTools.ECS
 			{
 				entityIDToStorageIndex[kvp.Key] = kvp.Value;
 			}
+
+			nextID = serializedComponentStorage.Count;
 		}
 	}
 }
diff --git a/src/ComponentStorageState.cs b/src/ComponentStorageState.cs
index dd98063..dfc3e52 100644
--- a/src/ComponentStorageState.cs
+++ b/src/ComponentStorageState.cs
@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Runtime.InteropServices;
 
 namespace MoonTools.ECS
 {
@@ -11,12 +10,12 @@ namespace MoonTools.ECS
 		public byte[] EntityIDs;
 		public byte[] Components;
 
-        public static ComponentStorageState Create<TComponent>(int count)
+        public unsafe static ComponentStorageState Create<TComponent>(int count) where TComponent : unmanaged
         {
 			return new ComponentStorageState(
                 count,
-                count * Marshal.SizeOf<int>(),
-                count * Marshal.SizeOf<TComponent>()
+                count * sizeof(int),
+                count * sizeof(TComponent)
             );
 		}
 
diff --git a/src/FilterBuilder.cs b/src/FilterBuilder.cs
index 4ba782d..be75edc 100644
--- a/src/FilterBuilder.cs
+++ b/src/FilterBuilder.cs
@@ -23,14 +23,14 @@ namespace MoonTools.ECS
 			Excluded = excluded;
 		}
 
-		public FilterBuilder Include<TComponent>() where TComponent : struct
+		public FilterBuilder Include<TComponent>() where TComponent : unmanaged
 		{
 			ComponentDepot.Register<TComponent>();
 			Included.Add(typeof(TComponent));
 			return new FilterBuilder(ComponentDepot, Included, Excluded);
 		}
 
-		public FilterBuilder Exclude<TComponent>() where TComponent : struct
+		public FilterBuilder Exclude<TComponent>() where TComponent : unmanaged
 		{
 			ComponentDepot.Register<TComponent>();
 			Excluded.Add(typeof(TComponent));
diff --git a/src/FilterSignature.cs b/src/FilterSignature.cs
index 4857119..7508b36 100644
--- a/src/FilterSignature.cs
+++ b/src/FilterSignature.cs
@@ -7,8 +7,8 @@ namespace MoonTools.ECS
 	{
 		private const int HASH_FACTOR = 97;
 
-		public HashSet<Type> Included;
-		public HashSet<Type> Excluded;
+		public readonly HashSet<Type> Included;
+		public readonly HashSet<Type> Excluded;
 
 		public FilterSignature(HashSet<Type> included, HashSet<Type> excluded)
 		{
diff --git a/src/IndexableSet.cs b/src/IndexableSet.cs
index cf8f56c..6057cf1 100644
--- a/src/IndexableSet.cs
+++ b/src/IndexableSet.cs
@@ -1,10 +1,11 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Runtime.InteropServices;
 
 namespace MoonTools.ECS
 {
-	internal class IndexableSet<T> : IEnumerable<T> where T : notnull
+	internal class IndexableSet<T> : IEnumerable<T> where T : unmanaged
 	{
 		private Dictionary<T, int> indices;
 		private T[] array;
@@ -78,5 +79,38 @@ namespace MoonTools.ECS
 				yield return array[i];
 			}
 		}
+
+		public void Save(IndexableSetState<T> state)
+		{
+			state.Indices.Clear();
+			foreach (var (key, value) in indices)
+			{
+				state.Indices[key] = value;
+			}
+
+			ReadOnlySpan<byte> arrayBytes = MemoryMarshal.Cast<T, byte>(array);
+
+			if (arrayBytes.Length > state.Array.Length)
+			{
+				Array.Resize(ref state.Array, arrayBytes.Length);
+			}
+
+			arrayBytes.CopyTo(state.Array);
+
+			state.Count = Count;
+		}
+
+		public void Load(IndexableSetState<T> state)
+		{
+			indices.Clear();
+			foreach (var kvp in state.Indices)
+			{
+				indices[kvp.Key] = kvp.Value;
+			}
+
+			state.Array.CopyTo(MemoryMarshal.Cast<T, byte>(array));
+
+			Count = state.Count;
+		}
 	}
 }
diff --git a/src/IndexableSetState.cs b/src/IndexableSetState.cs
new file mode 100644
index 0000000..43b7b79
--- /dev/null
+++ b/src/IndexableSetState.cs
@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+
+namespace MoonTools.ECS
+{
+    public class IndexableSetState<T> where T : unmanaged
+    {
+		public int Count;
+		public Dictionary<T, int> Indices;
+		public byte[] Array;
+
+        public unsafe IndexableSetState(int count)
+        {
+			Count = count;
+			Indices = new Dictionary<T, int>(count);
+			Array = new byte[sizeof(T) * count];
+		}
+	}
+}
diff --git a/src/World.cs b/src/World.cs
index 1582846..a5dbe81 100644
--- a/src/World.cs
+++ b/src/World.cs
@@ -12,12 +12,12 @@
 			return EntityStorage.Create();
 		}
 
-		public void Set<TComponent>(Entity entity, in TComponent component) where TComponent : struct
+		public void Set<TComponent>(Entity entity, in TComponent component) where TComponent : unmanaged
 		{
 			ComponentDepot.Set(entity.ID, component);
 		}
 
-		public void Send<TMessage>(in TMessage message) where TMessage : struct
+		public void Send<TMessage>(in TMessage message) where TMessage : unmanaged
 		{
 			MessageDepot.Add(message);
 		}
@@ -27,6 +27,11 @@
 			MessageDepot.Clear();
 		}
 
+		public void DisableSerialization<TComponent>() where TComponent : unmanaged
+		{
+			ComponentDepot.DisableSerialization<TComponent>();
+		}
+
 		public ComponentDepotState Serialize()
 		{
 			var state = ComponentDepot.CreateState();