Stress FAIL - what do these messages mean?

[stresslinux]

looks like stress is writing to the ram disk (--hdd 20) of the bootet image.
you should mount a local hard drive(partition) e.g. to /mnt and change to that directory before starting stress, it will create some temporary files in that
directory (default size per file is 1GB), make sure there is enough space, with 20 hdd workers you will need 20GB free space