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I created a dynamically growing VirtualBox VDI and wanted to shrink its size after a long time of use. I briefly searched for the solution and it seems that the steps to do so are:

  1. Zero the free space (by creating a large file filled with zeros).
  2. Run vboxmanage modifyhd <VDI> --compact.

My problem here is that the VDI size (seen in the guest) is much larger than that of the partition where the VDI resides (e.g. 2TB VDI on a 320GB partition). Is the above method still doable in this case? If not, are there any other possible solutions?

Also, if VDI fails to support such an allocation scheme, will any other virtual disk formats do?

phuclv
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Cyker
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3 Answers3

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You can let VirtualBox automatically discard the unused blocks from the image when it receives the TRIM command (which is supported since VirtualBox 4.2) from the OS

  • Storage: implemented support for discarding unused image blocks through TRIM for SATA and IDE and UNMAP for SCSI when using VDI images

You'll need to use the --nonrotational on and --discard on options to set the disk image settings, something like

VBoxManage storageattach <VM name> --storagectl "SATA" --port 1 --device 0 --nonrotational on --discard on --medium path\to\file.vdi --type hdd

  • --discard

    Enables the auto-discard feature for a virtual hard disks. This specifies that a VDI image will be shrunk in response to the trim command from the guest OS. The following requirements must be met:

    • The disk format must be VDI.
    • The size of the cleared area must be at least 1 MB.
    • Oracle VM VirtualBox will only trim whole 1 MB blocks. The VDIs themselves are organized into 1 MB blocks, so this will only work if the space being trimmed is at least a 1 MB contiguous block at a 1 MB boundary. On Windows, occasional defragmentation with defrag.exe /D, or on Linux running btrfs filesystem defrag as a background cron job may be beneficial.

You can also open the *.vbox configuration file and add discard="true" nonrotational="true" to the <AttachedDevice> tag

<AttachedDevice nonrotational="true" discard="true" type="HardDisk" port="1" device="0">
    <Image uuid="{3836a042-a83e-4000-9a59-e95ad65162ce}"/>
</AttachedDevice>

More information can be found in

phuclv
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The vboxmanage modifyhd (or modifymedium on more recent vbox versions) can compact the vdi file. A dynamic disk can be created as you mention like 2TB in a 350Gb partition, this kind of disk increases its size when you use more space in the virtual machine until there is no available space on the physical disk. Performing a compact doesn't get the full size of the vdi, you need to follow this process to compact the vdi file as the size you can see on the guest:

How to compact VirtualBox's VDI file size?

phuclv
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Genaro Morales
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I don't think that the VDI size itself can be bigger than the partition size where it resides. Do you mean that the 2TB is the dynamic size allocated to the VDI?

If yes, then i think you still can do it. But, you miss one step after creating a large zero-fill file. You need to delete it, and then you just power off the VM and then compact it.

Bento
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