How to Write VM Configuration Files in Ozone vmparser

Learn how to use vmparser writer functions to update VM config sections on disk without overwriting unrelated fields using the read-merge-write pattern.

Ozone’s vmparser writer functions follow a read-merge-write pattern: each function reads the existing vms/<vmname>.json file, replaces only the target section with the new value, then serialises the full VM struct back to disk using json.MarshalIndent. This means calling VMParserWriterCPU to update CPU settings will never touch the disk, memory, or network sections of the same file.

Import

import "github.com/stratosphere-ve/ozone/vmparser"

Writer functions are safe to call in sequence. Because each one reads the current file before writing, successive calls accumulate changes — no section is silently reset by a later write.

The vms/ directory must exist before calling any writer function. The functions do not create the directory automatically. If it is absent, os.WriteFile will return an error and the write will fail.

---

Writing by resource

VMInfo

Update the vm identity section. The VM name is derived from info.Name, so the file path is vms/<info.Name>.json.

info := vmparser.VMInfo{
    Name:      "web-01",
    UUID:      "550e8400-e29b-41d4-a716-446655440000",
    CreatedAt: "2024-06-01T12:00:00Z",
}

if err := vmparser.VMParserWriterInfo(info); err != nil {
    log.Fatalf("failed to write VMInfo: %v", err)
}

Signature: VMParserWriterInfo(info VMInfo) error

Note that this writer takes a VMInfo value directly (not a vmname string) because the name is embedded in the struct.

CPU

Update the cpu section for a named VM while preserving all other sections.

cpu := vmparser.CPU{
    Cores: 8,
    Type:  "host",
}

if err := vmparser.VMParserWriterCPU("web-01", cpu); err != nil {
    log.Fatalf("failed to write CPU config: %v", err)
}

Signature: VMParserWriterCPU(vmname string, cpu CPU) error

Memory

Update the memory section for a named VM.

memory := vmparser.Memory{
    SizeMB: 4096,
}

if err := vmparser.VMParserWriterMemory("web-01", memory); err != nil {
    log.Fatalf("failed to write memory config: %v", err)
}

Signature: VMParserWriterMemory(vmname string, memory Memory) error

Network

Update the network section for a named VM.

network := vmparser.Network{
    Interface: "virbr0",
    Type:      "virtio",
    MAC:       "52:54:00:ab:cd:ef",
}

if err := vmparser.VMParserWriterNetwork("web-01", network); err != nil {
    log.Fatalf("failed to write network config: %v", err)
}

Signature: VMParserWriterNetwork(vmname string, network Network) error

Disk

Update the disk section for a named VM.

disk := vmparser.Disk{
    Name:   "web-01-disk",
    Path:   "/var/lib/ozone/images/web-01.qcow2",
    SizeGB: 40.0,
    Format: "qcow2",
}

if err := vmparser.VMParserWriterDisk("web-01", disk); err != nil {
    log.Fatalf("failed to write disk config: %v", err)
}

Signature: VMParserWriterDisk(vmname string, disk Disk) error

---

Full working example

The following snippet provisions a new VM configuration from scratch by writing each resource section in sequence. Because each writer preserves unrelated fields, the order does not matter.

package main

import (
    "log"

    "github.com/stratosphere-ve/ozone/vmparser"
)

func main() {
    const vmname = "web-01"

    // 1. Write identity information
    if err := vmparser.VMParserWriterInfo(vmparser.VMInfo{
        Name:      vmname,
        UUID:      "550e8400-e29b-41d4-a716-446655440000",
        CreatedAt: "2024-06-01T12:00:00Z",
    }); err != nil {
        log.Fatalf("write VMInfo: %v", err)
    }

    // 2. Configure CPU
    if err := vmparser.VMParserWriterCPU(vmname, vmparser.CPU{
        Cores: 4,
        Type:  "host",
    }); err != nil {
        log.Fatalf("write CPU: %v", err)
    }

    // 3. Configure memory
    if err := vmparser.VMParserWriterMemory(vmname, vmparser.Memory{
        SizeMB: 2048,
    }); err != nil {
        log.Fatalf("write memory: %v", err)
    }

    // 4. Configure network
    if err := vmparser.VMParserWriterNetwork(vmname, vmparser.Network{
        Interface: "virbr0",
        Type:      "virtio",
        MAC:       "52:54:00:ab:cd:ef",
    }); err != nil {
        log.Fatalf("write network: %v", err)
    }

    // 5. Configure disk
    if err := vmparser.VMParserWriterDisk(vmname, vmparser.Disk{
        Name:   "web-01-disk",
        Path:   "/var/lib/ozone/images/web-01.qcow2",
        SizeGB: 20.0,
        Format: "qcow2",
    }); err != nil {
        log.Fatalf("write disk: %v", err)
    }

    log.Println("VM configuration written successfully.")
}