Comment 8 for bug 981104

Setting this as a application configuration option is a fairly common practice for applications.

example: http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_tmpdir

While this could be accomplished by adding ( TMPDIR|TEMP|TMP = /some/tmpdir ) to /etc/environment for Ubuntu and /etc/profile or /etc/profile.d/nova.sh for RHEL based systems.. I would hesitate to use that method as a systems administrator. I would be worried about what unintended consequences i would end up with by taking this route.. as the expected behavior for the rest of the system is to utilize /tmp for temp dirs/files..

Given that this function in nova can have huge space ramifications, the sane (from an ops perspective) approach is not to modify the global environment variables, but allow for fine grained administrator control by providing a configuration option.