I assume this means there's a max cache size set somewhere. There's plenty of room left on our fs (hundreds of GB) so Zope isn't being allowed to use it at the moment. I'm asuming this is a Zope rather than local filesystem issue, but maybe I'm wrong; if it is a Zope issue, how I can increase standard cache size? We're not using a cache manager, yet...
None of the above ;-) It's most likely that your temp directory is mounted on a small partition. Trying setting the envar TMPDIR to point at a directory on "a large" partition before starting Zope. That is, you're not running into a cache size problem, you're running into that Zope is creating a large temp file on a partition without enough space to hold it.
Heh, it worked a treat! Blinkered as I was, I never thought to check the system temp dir; df-ing our filesystem tells me that today there is only 174M free on the root partition, where default /tmp is. That magical figure explains why I was able to download a 170M file but not a 180M file. Sorted. Big big thanks, Chris