ecryptfs.extend-file-random.sh.btrfs fails with "Failed to write 32 bytes, position nnnn: No space left on device"

This happened no Quantal LTS-HWE: of the 10 job runs, 6 failed the same way. This was not observed on this version of the tests elsewhere.

01/10 12:57:29 DEBUG|base_utils:0077| Running 'which gcc'
01/10 12:57:29 DEBUG|base_utils:0114| [stdout] /usr/bin/gcc
01/10 12:57:29 DEBUG|base_utils:0077| Running 'tests/run_one.sh -K -t extend-file-random.sh -b 1000000 -D /mnt/image -l /mnt/lower -u /mnt/upper -f btrfs'
01/10 12:57:42 ERROR|base_utils:0114| [stderr] Failed to write 32 bytes, position 324112986: No space left on device
01/10 12:57:42 DEBUG|base_utils:0114| [stdout] extend-file-random FAIL

Here is the test attributes blurb from one of the runs:

{
    "distro-release": "Ubuntu 12.04",
    "distro-release-name": "precise",
    "environ": {
        "HOME": "/home/ubuntu",
        "LANG": "en_US.UTF-8",
        "LOGNAME": "ubuntu",
        "MAIL": "/var/mail/ubuntu",
        "PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games",
        "PWD": "/home/ubuntu",
        "SHELL": "/bin/bash",
        "SHLVL": "1",
        "SSH_CLIENT": "192.168.123.1 54143 22",
        "SSH_CONNECTION": "192.168.123.1 54143 192.168.123.132 22",
        "USER": "ubuntu",
        "_": "/bin/sh"
    },
    "kernel": "3.5.0-22.34~precise1",
    "platform": {
        "arch": {
            "bits": "64bit",
            "linkage": "ELF"
        },
        "hardware": "virtual",
        "hostname": "clean-precise-generic-amd64",
        "libc": {
            "lib": "glibc",
            "version": "2.7"
        },
        "machine": "x86_64",
        "proc": "x86_64"
    },
    "timestamp": "Thursday, 10. January 2013 17:33 UTC",
    "version": "0.0.12-1"
}