lib/storage: validate timestamps in the block only if they use encoding, which needs validation

This reduces CPU usage when there is no sense in validating timestamps.

This is a follow-up for 5fa9525498

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2998
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3011

lib/encoding/encoding.go

@@ -42,6 +42,20 @@ const (
 	MarshalTypeNearestDelta = MarshalType(6)
 )
 
+// NeedsValidation returns true if mt may need additional validation for silent data corruption.
+func (mt MarshalType) NeedsValidation() bool {
+	switch mt {
+	case MarshalTypeNearestDelta2,
+		MarshalTypeNearestDelta:
+		return true
+	default:
+		// Other types do not need additional validation,
+		// since they either already contain checksums (e.g. compressed data)
+		// or they are trivial and cannot be validated (e.g. const or delta const)
+		return false
+	}
+}
+
 // CheckMarshalType verifies whether the mt is valid.
 func CheckMarshalType(mt MarshalType) error {
 	if mt < 0 || mt > 6 {

lib/storage/block.go

@@ -271,7 +271,7 @@ func (b *Block) UnmarshalData() error {
 	if b.bh.PrecisionBits < 64 {
 		// Recover timestamps order after lossy compression.
 		encoding.EnsureNonDecreasingSequence(b.timestamps, b.bh.MinTimestamp, b.bh.MaxTimestamp)
-	} else {
+	} else if b.bh.TimestampsMarshalType.NeedsValidation() {
 		// Ensure timestamps are in the range [MinTimestamp ... MaxTimestamps] and are ordered.
 		if err := checkTimestampsBounds(b.timestamps, b.bh.MinTimestamp, b.bh.MaxTimestamp); err != nil {
 			return err