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pkg/disk.yaml: add an option to not grow root #2524

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bd23b5d
pkg/disk.yaml: add an option to not grow root
jbtrystram Jul 10, 2026
c3343a0
pkg/disk/parition_table.go typo fix
jbtrystram Jul 10, 2026
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36 changes: 36 additions & 0 deletions doc/20-advanced/20-bootc/05-sources-of-configuration.md
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Expand Up @@ -69,6 +69,42 @@ partition_table:

- `type`, an `enum` that can be `gpt` or `dos` and sets the partition table format to use.
- `partitions`, a list of objects each of which represents a partition.
- `size`, an *optional* string with units to set the overall disk size (e.g. `"10 GiB"`). If omitted the disk will be sized to fit all partitions.
- `grow_root_to_fill_disk`, an *optional* boolean (defaults to `true`). When `true` (or omitted), the partition containing the root filesystem (`/`) is automatically grown to fill any remaining disk space. Set to `false` to keep the root partition at its specified size, leaving unallocated space on the disk. This is useful when the image is expected to be grown at first boot (e.g. via `cloud-utils-growpart`) or when you want a compact disk image.

Here's an example using `grow_root_to_fill_disk: false` to produce a 10 GiB disk where the root partition stays at 4 GiB, leaving the remaining space unallocated:

```yaml
mount_configuration: "units"
partition_table:
type: "gpt"
size: "10 GiB"
grow_root_to_fill_disk: false
partitions:
- size: "1 MiB"
type: "21686148-6449-6e6F-744e-656564454649"
bootable: true
- size: "200 MiB"
type: "c12a7328-f81f-11d2-ba4b-00a0c93ec93b"
payload_type: "filesystem"
payload:
type: "vfat"
mountpoint: "/boot/efi"
label: "ESP"
fstab_options: "defaults,uid=0,gid=0,umask=077,shortname=winnt"
fstab_freq: 0
fstab_passno: 2
- size: "4 GiB"
type: "0fc63daf-8483-4772-8e79-3d69d8477de4"
payload_type: "filesystem"
payload:
type: "ext4"
label: "root"
mountpoint: "/"
fstab_options: "defaults"
fstab_freq: 0
fstab_passno: 0
```

#### Partitions

Expand Down
38 changes: 28 additions & 10 deletions pkg/disk/partition_table.go
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Expand Up @@ -47,6 +47,12 @@ type PartitionTable struct {
// Dictates if certain bits and bobs are required or not; uses the default
// policy if not set.
Policy *PartitionTablePolicy `json:"policy,omitempty" yaml:"policy,omitempty"`

// Controls whether the root partition (the one containing "/") is
// grown to fill the remaining disk space during relayout. Defaults
// to true (nil means true) to preserve backward compatibility. Set
// to false to keep the root partition at its specified size.
GrowRootToFillDisk *bool `json:"grow_root_to_fill_disk,omitempty" yaml:"grow_root_to_fill_disk,omitempty"`
Comment on lines +51 to +55

@achilleas-k achilleas-k Jul 15, 2026

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I don't really like booleans defaulting to true. We do have it in a few places, but that's usually when interacting with external configs and their defaults. If we're making our own, I'd rather avoid it.

Negative bools (like DisableX = true) are also a bit tricky and error prone. But this whole discussion is moot anyway.

@jbtrystram jbtrystram Jul 15, 2026

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I made it default to true so it keep the same existing behavior when not specified, to not change people that haven't changed their configs.

}

type PartitionTablePolicy struct {
Expand All @@ -56,6 +62,15 @@ type PartitionTablePolicy struct {
EnsureXBOOTLDR bool `json:"ensure_xbootldr" yaml:"ensure_xbootldr"`
}

// growRootToFillDisk returns whether the root partition should be grown
// to fill remaining disk space. Defaults to true when not explicitly set.
func (pt *PartitionTable) growRootToFillDisk() bool {
if pt.GrowRootToFillDisk == nil {
return true
}
return *pt.GrowRootToFillDisk
}

var _ = MountpointCreator(&PartitionTable{})

// Offset describes the offset as a "size", this allows us to reuse
Expand Down Expand Up @@ -130,8 +145,9 @@ func NewDefaultPartitionTablePolicy() *PartitionTablePolicy {
//
// In the case of raw partitioning (no LVM and no Btrfs), the partition
// containing the root filesystem is grown to fill any left over space on the
// partition table. Logical Volumes are not grown to fill the space in the
// Volume Group since they are trivial to grow on a live system.
// partition table, unless the base partition table has GrowRootToFillDisk set
// to false. Logical Volumes are not grown to fill the space in the Volume
// Group since they are trivial to grow on a live system.
func NewPartitionTable(basePT *PartitionTable, mountpoints []blueprint.FilesystemCustomization, imageSize datasizes.Size, mode partition.PartitioningMode, architecture arch.Arch, requiredSizes map[string]datasizes.Size, defaultFs string, rng *rand.Rand) (*PartitionTable, error) {
newPT := basePT.Clone().(*PartitionTable)

Expand Down Expand Up @@ -237,6 +253,7 @@ func (pt *PartitionTable) Clone() Entity {
AbsoluteStartOffset: pt.AbsoluteStartOffset,
AlignFooter: pt.AlignFooter,
Policy: pt.Policy,
GrowRootToFillDisk: common.ClonePtr(pt.GrowRootToFillDisk),
}

for idx, partition := range pt.Partitions {
Expand Down Expand Up @@ -519,7 +536,8 @@ func (pt *PartitionTable) applyCustomization(mountpoints []blueprint.FilesystemC
// Dynamically calculate and update the start point for each of the existing
// partitions. Adjusts the overall size of image to either the supplied value
// in `size` or to the sum of all partitions if that is larger. Will grow the
// root partition if there is any empty space. Returns the updated start point.
// root partition if there is any empty space, unless GrowRootToFillDisk is
// set to false. Returns the updated start point.
func (pt *PartitionTable) relayout(size datasizes.Size) uint64 {
header := pt.HeaderSize()
footer := datasizes.Size(0)
Expand Down Expand Up @@ -562,11 +580,12 @@ func (pt *PartitionTable) relayout(size datasizes.Size) uint64 {
root := &pt.Partitions[rootIdx]
root.Start = start
root.fitTo(root.Size)
root.Size = pt.AlignUp(root.Size)

// add the extra padding specified in the partition table
footer += datasizes.Size(pt.ExtraPadding)

// If the sum of all partitions is bigger then the specified size,
// If the sum of all partitions is bigger than the specified size,
Comment thread
jbtrystram marked this conversation as resolved.
// we use that instead. Grow the partition table size if needed.
end := pt.AlignUp(datasizes.Size(root.Start) + footer + root.Size)
if end > size {
Expand All @@ -577,12 +596,11 @@ func (pt *PartitionTable) relayout(size datasizes.Size) uint64 {
pt.Size = datasizes.Size(size)
}

// If there is space left in the partition table, grow root
root.Size = pt.Size - datasizes.Size(root.Start)

// Finally we shrink the last partition, i.e. the root partition,
// to leave space for the footer, e.g. the secondary GPT header.
root.Size -= footer
if pt.growRootToFillDisk() {
// Grow root to fill remaining disk space, leaving room
// for the footer (e.g. the secondary GPT header).
root.Size = pt.Size - datasizes.Size(root.Start) - footer
}

// Sort partitions by start sector
pt.sortPartitions()
Expand Down
158 changes: 158 additions & 0 deletions pkg/disk/partition_table_internal_test.go
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Expand Up @@ -4,6 +4,7 @@ import (
"fmt"
"testing"

"github.com/osbuild/image-builder/internal/common"
"github.com/osbuild/image-builder/pkg/datasizes"
"github.com/stretchr/testify/assert"
)
Expand Down Expand Up @@ -889,6 +890,163 @@ func TestRelayout(t *testing.T) {
},
},
},
"no-grow-dos": {
pt: &PartitionTable{
Type: PT_DOS,
Size: 100 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Size: 10 * MiB,
},
{
Payload: &Filesystem{
Mountpoint: "/",
},
Size: 20 * MiB,
},
},
},
size: 100 * MiB,
expected: &PartitionTable{
Type: PT_DOS,
Size: 100 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Start: 1 * MiB,
Size: 10 * MiB,
},
{
Payload: &Filesystem{
Mountpoint: "/",
},
Start: 11 * MiB,
Size: 20 * MiB, // does NOT grow to fill
},
},
},
},
"no-grow-gpt": {
pt: &PartitionTable{
Type: PT_GPT,
Size: 100 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Size: 10 * MiB,
},
{
Payload: &Filesystem{
Mountpoint: "/",
},
Size: 20 * MiB,
},
},
},
size: 100 * MiB,
expected: &PartitionTable{
Type: PT_GPT,
Size: 100 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Start: 1 * MiB,
Size: 10 * MiB,
},
{
Payload: &Filesystem{
Mountpoint: "/",
},
Start: 11 * MiB,
Size: 20 * MiB, // does NOT grow to fill
},
},
},
},
"no-grow-gpt-disk-grows-to-fit-partitions": {
// When the disk size is too small to fit all partitions,
// the disk grows to fit even with no-grow set.
pt: &PartitionTable{
Type: PT_GPT,
Size: 10 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Size: 10 * MiB,
},
{
Payload: &Filesystem{
Mountpoint: "/",
},
Size: 20 * MiB,
},
},
},
size: 10 * MiB,
expected: &PartitionTable{
Type: PT_GPT,
Size: 32 * MiB, // grown to fit: 1 MiB header + 10 MiB + 20 MiB + footer, aligned up
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Start: 1 * MiB,
Size: 10 * MiB,
},
{
Payload: &Filesystem{
Mountpoint: "/",
},
Start: 11 * MiB,
Size: 20 * MiB, // does NOT grow beyond specified size
},
},
},
},
"no-grow-gpt-root-first": {
pt: &PartitionTable{
Type: PT_GPT,
Size: 100 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Size: 10 * MiB,
Payload: &Filesystem{
Mountpoint: "/",
},
},
{
Size: 20 * MiB,
},
{
Size: 30 * MiB,
},
},
},
size: 100 * MiB,
expected: &PartitionTable{
Type: PT_GPT,
Size: 100 * MiB,
GrowRootToFillDisk: common.ToPtr(false),
Partitions: []Partition{
{
Start: 1 * MiB,
Size: 20 * MiB,
},
{
Start: 21 * MiB,
Size: 30 * MiB,
},
{
Start: 51 * MiB, // root gets moved to last position
Size: 10 * MiB, // does NOT grow beyond specified size
Payload: &Filesystem{
Mountpoint: "/",
},
},
},
},
},
}

for name := range testCases {
Expand Down
61 changes: 61 additions & 0 deletions pkg/disk/partition_table_yaml_test.go
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Expand Up @@ -7,6 +7,7 @@ import (
"github.com/stretchr/testify/require"
"go.yaml.in/yaml/v3"

"github.com/osbuild/image-builder/internal/common"
"github.com/osbuild/image-builder/pkg/datasizes"
"github.com/osbuild/image-builder/pkg/disk"
)
Expand Down Expand Up @@ -196,3 +197,63 @@ partition_table:
}
assert.Equal(t, expected, ptWrapper.PartitionTable)
}

func TestPartitionTableUnmarshalYAMLGrowRootToFillDisk(t *testing.T) {
tests := map[string]struct {
yaml string
expected *bool
}{
"absent": {
yaml: `
partition_table:
type: "gpt"
partitions:
- size: "1 MiB"
payload_type: "filesystem"
payload:
type: "ext4"
mountpoint: "/"
`,
expected: nil,
},
"true": {
yaml: `
partition_table:
type: "gpt"
grow_root_to_fill_disk: true
partitions:
- size: "1 MiB"
payload_type: "filesystem"
payload:
type: "ext4"
mountpoint: "/"
`,
expected: common.ToPtr(true),
},
"false": {
yaml: `
partition_table:
type: "gpt"
grow_root_to_fill_disk: false
partitions:
- size: "1 MiB"
payload_type: "filesystem"
payload:
type: "ext4"
mountpoint: "/"
`,
expected: common.ToPtr(false),
},
}

for name, tc := range tests {
t.Run(name, func(t *testing.T) {
var ptWrapper struct {
PartitionTable disk.PartitionTable `yaml:"partition_table"`
}
err := yaml.Unmarshal([]byte(tc.yaml), &ptWrapper)
require.NoError(t, err)
assert.Equal(t, tc.expected, ptWrapper.PartitionTable.GrowRootToFillDisk)
})
}
}
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