[Libguestfs] virt-builder disk space

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: libguestfs: trace: get_cachedir = "/var/tmp" /usr/bin/supermin-helper: ext2fs_read_inode: Illegal inode number This works fine: export TMPDIR=/mnt virt-builder --cache /mnt fedora-20 More output attached. I love virt-builder btw.