So what does your "shmpages" and your sysctl inside the container claim to be the max shared memory available. If you set the shmmax, but the shmpages in your container config is to short Postgres maybe has no way to find out beside crashing (http://openvz.org/Postgresql_and_shared_memory).
shmpages is "unlimited" in the container
What is did do is just set the the shmmax really high on the container sysctl. That way it wouldn't matter that there is less shared memory than there's supposed to be.
I have not yet been able to reproduce the error.
Though this does make me feel a little stupid, it seems to make sense and work too.
I'll post back if the error does come up again.
Thanks a lot for all the help,
WBL
-- "Quality comes from focus and clarity of purpose" -- Mark Shuttleworth
