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OS X filesystem does not support seeking ahead to create sparse files.
The test is supposed to skip the LargeMmapTests on OS X and Windows with (line 679 of test_mmap.py):
if sys.platform[:3] == 'win' or sys.platform == 'darwin':
 requires('largefile', 'test requires %s bytes and a long time to run' % str(0x180000000))
Perhaps on the Snow Leopard buildbot something causes this line not to become true (what does sys.platform give on the buildbot)?
As for why its creating the file slowly (encrypted fs?), I don't know but the test shouldn't be running anyway...