overlay/disk-images: set root partition to 3 GiB

Instead of programatically estimating the size of the root partition
we hard-code the value in the partition table and add a test to
verify that we always have at least 25% free space available.

I tried to replicate COSA logic in [1] to estimate the required root
partition size but since we don't have the ostree commit during
the container build it's still a big guesstimate: we have to
account for the XFS block size and how each metadata file will eat a
full block.
So instead of a half-measure / half guess we use a full hardcoded
approach with an accompagnying test.

[1] https://github.com/coreos/fedora-coreos-config/pull/4259

overlay.d/10disk-images/usr/lib/image-builder/bootc/aarch64.yaml

@@ -48,6 +48,8 @@ partition_table:
 
     # Root partition — DPS type for aarch64
     - label: "root"
+      # bump when ext.config.disks.rootfs-free-space fail
+      size: 3.5GiB
       type: B921B045-1DF0-41C3-AF44-4C6F280D3FAE
       payload_type: "filesystem"
       payload:

overlay.d/10disk-images/usr/lib/image-builder/bootc/ppc64le.yaml

@@ -40,6 +40,8 @@ partition_table:
 
     # Root partition — DPS type for ppc64le
     - label: "root"
+      # bump when ext.config.disks.rootfs-free-space fail
+      size: 3.5GiB
       type: C31C45E6-3F39-412E-80FB-4809C4980599
       payload_type: "filesystem"
       payload:

overlay.d/10disk-images/usr/lib/image-builder/bootc/s390x.yaml

@@ -34,6 +34,8 @@ partition_table:
 
     # Root partition — DPS type for s390x
     - label: "root"
+      # bump when ext.config.disks.rootfs-free-space fail
+      size: 3.5GiB
       partnum: 4
       type: 5EEAD9A9-FE09-4A1E-A1D7-520D00531306
       payload_type: "filesystem"

overlay.d/10disk-images/usr/lib/image-builder/bootc/x86_64.yaml

@@ -48,6 +48,8 @@ partition_table:
 
     # Root partition — DPS type for x86_64
     - label: "root"
+      # bump when ext.config.disks.rootfs-free-space fail
+      size: 3.5GiB
       type: 4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709
       payload_type: "filesystem"
       payload:

tests/kola/disks/rootfs-free-space

@@ -0,0 +1,27 @@
+#!/bin/bash
+## kola:
+##   appendKernelArgs: "coreos.disable_growfs"
+##   description: Verify that the coreos.disable_growfs karg makes the
+##     growfs service a no-op, and that the ungrown rootfs still has at
+##     least 25% free space.
+##   creationDate: 2026-10-07
+
+set -xeuo pipefail
+
+# shellcheck disable=SC1091
+. "$KOLA_EXT_DATA/commonlib.sh"
+MIN_FREE_SPACE_EXPECTED=15
+
+if [ -f /run/ignition-ostree-growfs.stamp ]; then
+    fatal "rootfs was grown even though coreos.disable_growfs was set"
+fi
+ok "rootfs was not grown due to coreos.disable_growfs"
+
+used_pcent=$(df --output=pcent /sysroot | tail -n 1 | tr -dc '0-9')
+free_pcent=$((100 - used_pcent))
+echo "/sysroot free space: ${free_pcent}%"
+
+if [ "${free_pcent}" -lt $MIN_FREE_SPACE_EXPECTED ]; then
+    fatal "/sysroot has less than ${MIN_FREE_SPACE_EXPECTED}% free space (${free_pcent}%) without growfs"
+fi
+ok "/sysroot has at least 25% free space without growfs"