qm set disk size

Daniel Parker

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28-02-2025

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Creating a virtual machine (VM) with QEMU often involves specifying the disk size. This guide explains how to set the disk size during VM creation, and how to adjust it later if needed. We'll cover various methods and considerations for optimal performance and storage management.

Understanding QEMU Disk Images

Before diving into setting the disk size, understanding QEMU's disk image formats is crucial. Common formats include:

• qcow2: A popular choice, offering features like efficient storage and the ability to dynamically grow the disk.
• raw: A simple format, offering good performance but using the full allocated space immediately.
• vmdk: Compatible with VMware, useful for migrating VMs between QEMU and VMware.
• vdi: Used by VirtualBox, again allowing for interoperability.

The choice of format influences how the disk size setting impacts storage usage. Dynamically allocated formats like qcow2 only consume storage as the VM uses it, while static formats like raw use the full allocated space from the start.

Setting Disk Size During VM Creation

The most straightforward way to manage disk size is during VM creation using the qemu-img command. This allows for precise control and avoids potential issues later on.

Using qemu-img to Create a Disk Image

The following command creates a 20GB qcow2 disk image named mydisk.qcow2:

qemu-img create -f qcow2 mydisk.qcow2 20G

Replace 20G with your desired size. Units can be B (bytes), K (kilobytes), M (megabytes), G (gigabytes), or T (terabytes). You can specify a different format using the -f option. For example, to create a raw image:

qemu-img create -f raw mydisk.raw 20G

Using QEMU's Command-Line Options

When starting a VM, you can specify the disk image using the -drive option:

qemu-system-x86_64 -m 2G -drive file=mydisk.qcow2,if=virtio

This command starts an x86_64 VM with 2GB of RAM and uses mydisk.qcow2 as the virtual hard disk. The if=virtio part specifies the interface type (virtio is generally recommended for performance).

Adjusting Disk Size After VM Creation

Sometimes, you might need to resize a disk after the VM is already created. This process depends on the disk format and whether the VM is running.

Expanding a Disk Image

For dynamic formats like qcow2, expanding the size is relatively simple using qemu-img:

qemu-img resize mydisk.qcow2 +10G

This adds 10GB to the existing mydisk.qcow2 image. However, you'll typically need to resize the partition inside the VM's operating system after this to make the additional space usable.

Shrinking a Disk Image (qcow2)

Shrinking a qcow2 disk is more complex and requires the VM to be shut down. Use the -c option with qemu-img for the shrinking process. Note that this only reclaims unused space within the virtual disk.

qemu-img convert -c -O qcow2 mydisk.qcow2 mydisk_shrunk.qcow2

This will create a new qcow2 image mydisk_shrunk.qcow2 that's compacted to eliminate unused space. You then need to replace the original image with the shrunk version.

Caution: Shrinking a disk image can potentially lead to data loss if not done correctly. Always back up your data before attempting this.

Resizing Raw Images

Resizing raw images is generally not recommended, especially shrinking them. Raw images are fixed-size, and resizing them requires creating a new, larger (or smaller) image and copying the data. This is a time-consuming process.

Best Practices and Considerations

• Choose the right format: qcow2 is a good general-purpose choice. raw can be faster but less flexible.
• Over-provisioning: Allocate slightly more space than initially anticipated to accommodate future growth.
• Regular backups: Protect your data with regular backups.
• Guest OS resizing: Remember that expanding the disk image doesn't automatically expand the partition inside the guest OS. You must do that within the guest operating system.

By understanding these methods and considerations, you can effectively manage disk size for your QEMU virtual machines, optimizing performance and storage usage. Remember to always back up your data before making any significant changes to your virtual disks.