For the second time in the one month, I ran into this problem. Here's how I solved it. (As a reminder for myself the next time I need it...)

During a migration of the Plone site of one of our customers, I ran out of space on /tmp. Frustrating, especially if you've been waiting for quite some time already. If you've ran into this problem or expect that you will not have enough space in the tmp directory, start the instance like this:

$ TMPDIR=/path/to/alternative/tmp/dir bin/instance start

Now the temporary stuff will be written to /path/to/alternative/tmp/dir instead of /tmp.

Update (2009-05-30):

After the comment by Wouter, I tried to include the alternative tmp directory in the buildout configuration. I'm under the impression that just adding the lines in the [instance] section doesn't work directly: the <buildout>/var/tmp directory doesn't seem to be generated when running "bin/buildout" and thus Zope doesn't use it. I solved this by using the ore.recipe.fs package:

[parts]
    ...
    tmp

[tmp]
recipe = ore.recipe.fs:mkdir
path = ${buildout:directory}/var/tmp

[instance]
...
environment-vars =
    TMP ${tmp:path}

If anyone knows a better way (Wouter?) please leave a comment...