How do I shrink a dynamically growing VDI disk from VirtualBox?

Question

My dynamically growing virtual disks (VDI) from various Ubuntu installations in Virtual Box are actually steadily "growing" indeed.

Deleting of files in the virtual machine does not free the space my VDI file took from the host hard disk space.

How do I shrink these files to give the unused hard disk space back to the host?

Answer 1

A dynamically growing Virtual Box virtual hard drive file is capable of growing on demand of the virtual machine up to the maximum size we defined in setting up this file. It does not however free the space of files we deleted in the guest OS.

In case the disk had grown too much we can compact it again, provided the unused space is filled with zeros, and the drive is in VDI format.

For an ext2 to ext 4 filesystem this can be done from the Ubuntu guest with the command line utility zerofree ^* .

zerofree /dev/sdxX

This needs the drive to not be in use, and to be unmounted. We therefore may have to bind it temporarily to another VM we had created for this purpose, or we need to boot an Ubuntu live environment on this VM.

Replace /dev/sdxX with the ext2, ext3 or ext4 formatted partition in question.

On more than one partition we my have to repeat zerofree for each partition. Keep in mind that by filling with zeros the virtual file will temporarily grow up to it's maximum size.

Once all unused drive space is filled with zeros we then shrink the drive with

VBoxManage modifyhd <name>.vdi --compact

This will considerably decrease the file size of our VDI file.

^{* In a Windows guest we can replace unused space with the utility sdelete.}

Answer 2

Here are some alternative ways to zero the free disk space (which can be the most time-consuming part). Afterwards, you will still have to run VBoxManage modifyhd <disk> --compact to compact the actual disk file.

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This command can zero the disk while it is unmounted:

apt-get install zerofree
zerofree -v /dev/sdxN

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Here's an example of how to get dd to periodically print output:

dd if=/dev/zero of=temp-file-on-disk bs=20M

# run this to get the process ID of the dd command
pgrep -l '^dd$'
# then put the process ID on the command line to "kill -USR1"
# this will tell dd to print the current status (in another terminal)
kill -USR1 $(pgrep -l '^dd$' | awk '{print $1}')

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Here's a dd-like program that will automatically print output periodically:

apt-get install dcfldd
dcfldd if=/dev/zero of=temp-file-on-disk bs=20M