diff --git a/scripts/raid-size-tests.py b/scripts/raid-size-tests.py
index 7b39cf72..0a28d985 100644
--- a/scripts/raid-size-tests.py
+++ b/scripts/raid-size-tests.py
@@ -7,16 +7,13 @@
 # raid devices (backed by sparse files in a tmpfs) and comparing their sizes
 # with the estimates. It must be run as root.
 
-import atexit
 import os
 import random
-import shutil
 import subprocess
 import sys
 import tempfile
 import uuid
 
-import attr
 
 from subiquity.models.filesystem import (
     align_down,
@@ -25,6 +22,9 @@ from subiquity.models.filesystem import (
     humanize_size,
     raidlevels,
     )
+from subiquity.models.tests.test_filesystem import (
+    FakeDev,
+    )
 
 
 tmpdir = tempfile.mkdtemp()
@@ -96,11 +96,6 @@ def get_real_raid_size(raid):
         stdout=subprocess.PIPE, encoding='ascii').stdout.strip())
 
 
-@attr.s
-class FakeDev:
-    size = attr.ib()
-
-
 def verify_size_ok(level, sizes):
     r = False
     try:
diff --git a/subiquity/models/filesystem.py b/subiquity/models/filesystem.py
index ab89878d..1f06c008 100644
--- a/subiquity/models/filesystem.py
+++ b/subiquity/models/filesystem.py
@@ -258,10 +258,15 @@ def calculate_data_offset(devsize):
     return data_offset
 
 
+def raid_device_sort(devices):
+    return sorted(devices, key=lambda d: d.id)
+
+
 # This this is tested against reality in ./scripts/get-raid-sizes.py
 def get_raid_size(level, devices):
     if len(devices) == 0:
         return 0
+    devices = raid_device_sort(devices)
     data_offset = calculate_data_offset(devices[0].size)
     sizes = [align_down(dev.size - data_offset) for dev in devices]
     min_size = min(sizes)
@@ -893,6 +898,13 @@ class Raid(_Device):
     name = attr.ib()
     raidlevel = attr.ib(converter=lambda x: raidlevels_by_value[x].value)
     devices = attributes.reflist(backlink="_constructed_device")
+
+    def serialize_devices(self):
+        # Surprisingly, the order of devices passed to mdadm --create
+        # matters (see get_raid_size) so we sort devices here the same
+        # way get_raid_size does.
+        return [d.id for d in raid_device_sort(self.devices)]
+
     spare_devices = attributes.reflist(backlink="_constructed_device")
 
     preserve = attr.ib(default=False)
diff --git a/subiquity/models/tests/test_filesystem.py b/subiquity/models/tests/test_filesystem.py
index c37672c1..af854468 100644
--- a/subiquity/models/tests/test_filesystem.py
+++ b/subiquity/models/tests/test_filesystem.py
@@ -19,6 +19,7 @@ import unittest
 import attr
 
 from subiquity.models.filesystem import (
+    attributes,
     Bootloader,
     dehumanize_size,
     DeviceAction,
@@ -108,14 +109,17 @@ class TestDehumanizeSize(unittest.TestCase):
                 self.assertEqual(expected_error, actual_error)
 
 
+@attr.s
+class FakeDev:
+
+    size = attr.ib()
+    id = attributes.idfield("fakedev")
+
+
 class TestRoundRaidSize(unittest.TestCase):
 
     def test_lp1816777(self):
 
-        @attr.s
-        class FakeDev:
-            size = attr.ib()
-
         self.assertLessEqual(
             get_raid_size("raid1", [FakeDev(500107862016)]*2),
             499972571136)