If data are available in persistent node, copy it from there instead …

…of storing in WAL

db.go

@@ -673,8 +673,9 @@ func (db *DB) processDeallocations(ctx context.Context, pipeReader *pipeline.Rea
 				case wal.RecordSet32:
 					wrIndex += 41
 				case wal.RecordSet:
-					wrIndex += 9
-					wrIndex += *(*uint16)(unsafe.Pointer(&walNode[wrIndex])) + 2
+					wrIndex += *(*uint16)(unsafe.Pointer(&walNode[wrIndex+9])) + 11
+				case wal.RecordCopy:
+					wrIndex += 19
 				case wal.RecordImmediateDeallocation, wal.RecordDelayedDeallocation:
 					nodeSnapshotID := *(*types.SnapshotID)(unsafe.Pointer(&walNode[wrIndex+1]))
 					oldNodeAddress := *(*types.NodeAddress)(unsafe.Pointer(&walNode[wrIndex+9]))
@@ -685,7 +686,7 @@ func (db *DB) processDeallocations(ctx context.Context, pipeReader *pipeline.Rea
 						return err
 					}
 				default:
-					panic("unrecognized record type")
+					panic(fmt.Sprintf("unrecognized record type at index %d", wrIndex))
 				}
 			}
 		}
@@ -731,16 +732,17 @@ func (db *DB) copyNodes(
 					case wal.RecordSet32:
 						wrIndex += 41
 					case wal.RecordSet:
-						wrIndex += 9
-						wrIndex += *(*uint16)(unsafe.Pointer(&walNode[wrIndex])) + 2
+						wrIndex += *(*uint16)(unsafe.Pointer(&walNode[wrIndex+9])) + 11
+					case wal.RecordCopy:
+						wrIndex += 19
 					case wal.RecordImmediateDeallocation, wal.RecordDelayedDeallocation:
 						oldNodeAddress := *(*types.NodeAddress)(unsafe.Pointer(&walNode[wrIndex+9]))
 						newNodeAddress := *(*types.NodeAddress)(unsafe.Pointer(&walNode[wrIndex+17]))
 						wrIndex += 25
 
 						copy(db.config.State.Bytes(newNodeAddress), db.config.State.Bytes(oldNodeAddress))
 					default:
-						panic("unrecognized record type")
+						panic(fmt.Sprintf("unrecognized record type at index %d", wrIndex))
 					}
 				}
 			}
@@ -766,26 +768,35 @@ func (db *DB) applyWALChanges(ctx context.Context, pipeReader *pipeline.Reader)
 			for wrIndex < types.NodeLength && wal.RecordType(walNode[wrIndex]) != wal.RecordEnd {
 				switch recordType := wal.RecordType(walNode[wrIndex]); recordType {
 				case wal.RecordSet1:
-					offset := *(*uint64)(unsafe.Pointer(&walNode[wrIndex+1]))
+					offset := *(*uintptr)(unsafe.Pointer(&walNode[wrIndex+1]))
 					*(*byte)(unsafe.Add(origin, offset)) = walNode[wrIndex+9]
 					wrIndex += 10
 				case wal.RecordSet8:
-					offset := *(*uint64)(unsafe.Pointer(&walNode[wrIndex+1]))
+					offset := *(*uintptr)(unsafe.Pointer(&walNode[wrIndex+1]))
 					copy(unsafe.Slice((*byte)(unsafe.Add(origin, offset)), 8), walNode[wrIndex+9:])
 					wrIndex += 17
 				case wal.RecordSet32:
-					offset := *(*uint64)(unsafe.Pointer(&walNode[wrIndex+1]))
+					offset := *(*uintptr)(unsafe.Pointer(&walNode[wrIndex+1]))
 					copy(unsafe.Slice((*byte)(unsafe.Add(origin, offset)), 32), walNode[wrIndex+9:])
 					wrIndex += 41
 				case wal.RecordSet:
-					offset := *(*uint64)(unsafe.Pointer(&walNode[wrIndex+1]))
+					offset := *(*uintptr)(unsafe.Pointer(&walNode[wrIndex+1]))
 					size := *(*uint16)(unsafe.Pointer(&walNode[wrIndex+9]))
 					copy(unsafe.Slice((*byte)(unsafe.Add(origin, offset)), size), walNode[wrIndex+11:])
 					wrIndex += size + 11
+				case wal.RecordCopy:
+					offsetDst := *(*uintptr)(unsafe.Pointer(&walNode[wrIndex+1]))
+					offsetSrc := *(*uintptr)(unsafe.Pointer(&walNode[wrIndex+9]))
+					size := *(*uint16)(unsafe.Pointer(&walNode[wrIndex+17]))
+
+					copy(unsafe.Slice((*byte)(unsafe.Add(origin, offsetDst)), size),
+						unsafe.Slice((*byte)(unsafe.Add(origin, offsetSrc)), size))
+
+					wrIndex += 19
 				case wal.RecordImmediateDeallocation, wal.RecordDelayedDeallocation:
 					wrIndex += 25
 				default:
-					panic("unrecognized record type")
+					panic(fmt.Sprintf("unrecognized record type at index %d", wrIndex))
 				}
 			}
 		}

space/space.go

@@ -600,8 +600,12 @@ func (s *Space[K, V]) splitDataNode(
 		}
 
 		newDataNodeItem := s.config.DataNodeAssistant.Item(newDataNode, s.config.DataNodeAssistant.ItemOffset(i))
-		if err := wal.Set2(walRecorder, tx, newNodePersistentAddress,
+		if err := wal.Copy(walRecorder, tx, newNodePersistentAddress, existingNodePointer.PersistentAddress,
 			newDataNodeItem, item,
+		); err != nil {
+			return err
+		}
+		if err := wal.Set1(walRecorder, tx, newNodePersistentAddress,
 			&newKeyHashes[i], &keyHashes[i],
 		); err != nil {
 			return err

wal/record.go

@@ -28,6 +28,9 @@ const (
 	// RecordSet records modification of variable-length slice.
 	RecordSet
 
+	// RecordCopy records a request to copy data from one place to another.
+	RecordCopy
+
 	// RecordImmediateDeallocation records node immediate deallocation.
 	RecordImmediateDeallocation
 
@@ -155,11 +158,11 @@ func Reserve[T comparable](
 	recorder *Recorder,
 	tx *pipeline.TransactionRequest,
 	dstNodeAddress qtypes.NodeAddress,
-	srcPointer *T,
+	dstPointer *T,
 ) (*T, error) {
 	var t T
 
-	offset := uintptr(dstNodeAddress*qtypes.NodeLength) + uintptr(unsafe.Pointer(srcPointer))%qtypes.NodeLength
+	offset := uintptr(dstNodeAddress*qtypes.NodeLength) + uintptr(unsafe.Pointer(dstPointer))%qtypes.NodeLength
 
 	switch size := unsafe.Sizeof(t); size {
 	case 1:
@@ -305,6 +308,33 @@ func Set4[T1, T2, T3, T4 comparable](
 	return nil
 }
 
+// Copy requests to copy data from one place to another.
+func Copy[T comparable](
+	recorder *Recorder, tx *pipeline.TransactionRequest, dstNodeAddress, srcNodeAddress qtypes.NodeAddress,
+	dst, src *T,
+) error {
+	var t T
+
+	size := uint16(unsafe.Sizeof(t))
+
+	offset := uintptr(unsafe.Pointer(dst)) % qtypes.NodeLength
+	offsetDst := uintptr(dstNodeAddress*qtypes.NodeLength) + offset
+	offsetSrc := uintptr(srcNodeAddress*qtypes.NodeLength) + offset
+
+	p, err := recorder.insert(tx, RecordCopy, 18)
+	if err != nil {
+		return err
+	}
+
+	*(*uintptr)(p) = offsetDst
+	*(*uintptr)(unsafe.Add(p, 8)) = offsetSrc
+	*(*uint16)(unsafe.Add(p, 16)) = size
+
+	*dst = *src
+
+	return nil
+}
+
 // Deallocate deallocate records node deallocation.
 // FIXME (wojciech): nodeSnapshotID is needed only in case of delayed deallocation.
 func Deallocate(