diff --git a/docs/kola/external-tests.md b/docs/kola/external-tests.md
index ac61dba8aa..8d2b82d250 100644
--- a/docs/kola/external-tests.md
+++ b/docs/kola/external-tests.md
@@ -231,7 +231,9 @@ In the example above, the test would only run if `--tag special` was provided.
 
 The `additionalDisks` key has the same semantics as the `--add-disk` argument
 to `qemuexec`. It is currently only supported on `qemu`. The `primaryDisk` key
-also supports the same syntax and controls the primary boot disk.
+also supports the same syntax and controls the primary boot disk. Only for the
+`primaryDisk` key, the size can be omitted (e.g. `:mpath`), in which case the
+qcow2 will not be resized.
 
 The `injectContainer` boolean if set will cause the framework to inject
 the ostree base image container into the target system; the path can be
diff --git a/mantle/platform/api/gcloud/compute.go b/mantle/platform/api/gcloud/compute.go
index 68a3b5b705..9e8e16434f 100644
--- a/mantle/platform/api/gcloud/compute.go
+++ b/mantle/platform/api/gcloud/compute.go
@@ -38,7 +38,7 @@ func (a *API) vmname() string {
 func ParseDisk(spec string, zone string) (*compute.AttachedDisk, error) {
 	var diskInterface string
 
-	size, diskmap, err := util.ParseDiskSpec(spec)
+	size, diskmap, err := util.ParseDiskSpec(spec, false)
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse disk spec %q: %w", spec, err)
 	}
diff --git a/mantle/platform/machine/qemu/cluster.go b/mantle/platform/machine/qemu/cluster.go
index 114daaf054..a16275a968 100644
--- a/mantle/platform/machine/qemu/cluster.go
+++ b/mantle/platform/machine/qemu/cluster.go
@@ -143,7 +143,7 @@ func (qc *Cluster) NewMachineWithQemuOptions(userdata *conf.UserData, options pl
 	var primaryDisk platform.Disk
 	if options.PrimaryDisk != "" {
 		var diskp *platform.Disk
-		if diskp, err = platform.ParseDisk(options.PrimaryDisk); err != nil {
+		if diskp, err = platform.ParseDisk(options.PrimaryDisk, true); err != nil {
 			return nil, errors.Wrapf(err, "parsing primary disk spec '%s'", options.PrimaryDisk)
 		}
 		primaryDisk = *diskp
diff --git a/mantle/platform/qemu.go b/mantle/platform/qemu.go
index 751bde03e6..6fe76da9fc 100644
--- a/mantle/platform/qemu.go
+++ b/mantle/platform/qemu.go
@@ -106,7 +106,7 @@ type Disk struct {
 	nbdServCmd     exec.Cmd // command to serve the disk
 }
 
-func ParseDisk(spec string) (*Disk, error) {
+func ParseDisk(spec string, allowNoSize bool) (*Disk, error) {
 	var channel string
 	sectorSize := 0
 	logicalSectorSize := 0
@@ -114,7 +114,7 @@ func ParseDisk(spec string) (*Disk, error) {
 	multipathed := false
 	var wwn uint64
 
-	size, diskmap, err := util.ParseDiskSpec(spec)
+	size, diskmap, err := util.ParseDiskSpec(spec, allowNoSize)
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse disk spec %q: %w", spec, err)
 	}
@@ -143,8 +143,12 @@ func ParseDisk(spec string) (*Disk, error) {
 		}
 	}
 
+	sizeStr := ""
+	if size > 0 {
+		sizeStr = fmt.Sprintf("%dG", size)
+	}
 	return &Disk{
-		Size:              fmt.Sprintf("%dG", size),
+		Size:              sizeStr,
 		Channel:           channel,
 		DeviceOpts:        serialOpt,
 		SectorSize:        sectorSize,
@@ -1302,7 +1306,7 @@ func (builder *QemuBuilder) AddDisk(disk *Disk) error {
 // AddDisksFromSpecs adds multiple secondary disks from their specs.
 func (builder *QemuBuilder) AddDisksFromSpecs(specs []string) error {
 	for _, spec := range specs {
-		if disk, err := ParseDisk(spec); err != nil {
+		if disk, err := ParseDisk(spec, false); err != nil {
 			return errors.Wrapf(err, "parsing additional disk spec '%s'", spec)
 		} else if err = builder.AddDisk(disk); err != nil {
 			return errors.Wrapf(err, "adding additional disk '%s'", spec)
diff --git a/mantle/util/common.go b/mantle/util/common.go
index 94acfac6b9..f5d7ea1477 100644
--- a/mantle/util/common.go
+++ b/mantle/util/common.go
@@ -125,10 +125,14 @@ func RunCmdTimeout(timeout time.Duration, cmd string, args ...string) error {
 
 // ParseDiskSpec converts a disk specification into a Disk. The format is:
 // <size>[:<opt1>,<opt2>,...], like ["5G:channel=nvme"]
-func ParseDiskSpec(spec string) (int64, map[string]string, error) {
+func ParseDiskSpec(spec string, allowNoSize bool) (int64, map[string]string, error) {
 	diskmap := map[string]string{}
 	split := strings.Split(spec, ":")
-	if split[0] == "" || (!strings.HasSuffix(split[0], "G")) {
+	if split[0] == "" {
+		if !allowNoSize {
+			return 0, nil, fmt.Errorf("no size provided in '%s'", spec)
+		}
+	} else if !strings.HasSuffix(split[0], "G") {
 		return 0, nil, fmt.Errorf("invalid size opt %s", spec)
 	}
 	var disksize string
@@ -149,10 +153,14 @@ func ParseDiskSpec(spec string) (int64, map[string]string, error) {
 	} else {
 		return 0, nil, fmt.Errorf("invalid disk spec %s", spec)
 	}
-	disksize = strings.TrimSuffix(disksize, "G")
-	size, err := strconv.ParseInt(disksize, 10, 32)
-	if err != nil {
-		return 0, nil, fmt.Errorf("failed to convert %q to int64: %w", disksize, err)
+	var size int64 = 0
+	if disksize != "" {
+		disksize = strings.TrimSuffix(disksize, "G")
+		var err error
+		size, err = strconv.ParseInt(disksize, 10, 32)
+		if err != nil {
+			return 0, nil, fmt.Errorf("failed to convert %q to int64: %w", disksize, err)
+		}
 	}
 	return size, diskmap, nil
 }