It seems to me there's no fundamental reason why there couldn't be
two VACUUMs running concurrently in a database. With the locking
we are doing now, it should be safe enough. So, I'd like to propose
that we get rid of the pg_vlock lock file. It doesn't have any useful
purpose but it does force manual intervention by the dbadmin to recover
if a VACUUM crashes :-(
Comments? Did I miss something about why we can't have more than one
vacuum process?
regards, tom lane