Hi,

I've got a feature request, it would be nice to have some friendly info in the case you don't have enough disk space on the default locations for virt-builder and supermin to run.

I was using virt-builder on the official Fedora AMI on EC2 which includes an awesome 2G root block device and, since RAM is at a premium in the smaller instance types, tmpfs is not used.

http://fedoraproject.org/en/get-fedora-options#clouds

However, EC2 also includes direct-attached "Instance Storage" scratch space such as 160G magnetic on m1.small or 32G ssd on m3.large (details at http://aws.amazon.com/ec2/pricing/ ) which cloud-init automatically mounts at /mnt. 

All you get output wise is :

virt-builder: libguestfs error: /usr/bin/supermin-helper exited with error status 1.

Even with LIBGUESTFS_DEBUG and LIBGUESTFS_TRACE supermin isn't super helpful but you can see lines such as:

This works fine:

export TMPDIR=/mnt

virt-builder --cache /mnt fedora-20

More output attached. I love virt-builder btw.