I forgot to mention: one important thing for buildd box to work was having a lot of memory (10G). We really need to fix the memory swapping issue, as this is making boxes with not a lot of memory (≤ 4G) very unstable as soon as you try to run ample operations such as installing latex-extra. Passing --sync=5 to the filesystems seems to be helping, probably by avoiding accumulating dirty pages, but that's just brown-tape-fixing.
Samuel
