pg_multixact not getting truncated

Hackers,

I'm looking at a couple of high-transaction-rate and high-FK-conflict
rate servers where pg_multixact has grown to be more than 1GB in size.
One such server doesn't appear to be having any notable issues with
vacuuming, and the oldest mxid on the system is about 47m old. VACUUM
FREEZEing the oldest databases did not cause the pg_multixact dir to get
smaller --- it may have even caused it to get larger.

Why would pg_multixact not be truncating?  Does it never truncate files
with aborted multixacts in them?  Might we have another multixact bug?

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com