while on Ubuntu kernel 2.6.28-14-generic Postgres 8.3.7-1 works fine, it doesn't start on Ubuntu 9.04, kernel 2.6.28-15-generic. Following is the log output of 'sudo /etc/init.d/postgresql-8.3 start':
Begin of log ==> 2009-09-08 08:17:52 CAT LOG: could not load root certificate file "root.crt": no SSL error reported 2009-09-08 08:17:52 CAT DETAIL: Will not verify client certificates. 2009-09-08 08:17:52 CAT FATAL: could not create shared memory segment: Invalid argument 2009-09-08 08:17:52 CAT DETAIL: Failed system call was shmget(key=5432001, size=38207488, 03600). 2009-09-08 08:17:52 CAT HINT: This error usually means that PostgreSQL's request for a shared memory segment exceeded your kernel's SHMMAX parameter. You can either reduce the request size or reconfigure the kernel with larger SHMMAX. To reduce the request size (currently 38207488 bytes), reduce PostgreSQL's shared_buffers parameter (currently 4096) and/or its max_connections parameter (currently 103). If the request size is already small, it's possible that it is less than your kernel's SHMMIN parameter, in which case raising the request size or reconfiguring SHMMIN is called for. The PostgreSQL documentation contains more information about shared memory configuration. <== end of log
As a workaround it's possible to boot into the "old" kernel.