I use VirtualBox [1] a lot as (local) virtualization software. It is a full-featured virtualization host, and supports multiple underlying disk image file types for guests.

One of those is VirtualBox' native Virtual Disk Image or VDI file type. An advantage of this type is that one can create a dynamically allocated image. This image will initially be very small and not occupy any space for unused virtual disk sectors, but will grow when a disk sector is written to for the first time. VirtualBox does this by checking for unused sectors.

However, this poses issues for disks with multiple partitions. If the last partition is say a (unused) swap partition, then VirtualBox does not automatically grow the underlying image. Even though the first partition is full, VirtualBox will not grow and therefore the host disk will be full without having reached its full potential.

To solve this issue, the machine needs to be partitioned using one big happy partition. Then VirtualBox will dynamically resize according to expectations.

I use packer [2] to prepare disk images for Debian, together with a preseed [3] file. Using preseeding to partition the disk is limited to what is supported by the partition tool …

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