korry wrote: > > I think the next question is -- how would the lock interface be used? > > We could acquire an exclusive lock on postmaster start (to make sure no > > backend is running), then reduce it to a shared lock. Every backend > > would inherit the shared lock. But the lock exchange is not guaranteed > > to be atomic so a new postmaster could start just after we acquire the > > lock and acquire the shared lock. It'd need to be complemented with > > another lock. > > You never need to reduce it to a shared lock. On postmaster startup, > try to lock the sentinel byte (one byte past the end-of-file). If you > can lock it, you know that no other postmaster has that byte locked. If > you can't lock it, another postmaster is running. It is an atomic > operation.
This doesn't work if the postmaster dies but a backend continues to run, which is arguably the most important case we need to protect against. > However, Tom may be correct about NFS locking, but I guess I'm surprised > that anyone would care :-) Quite a lot of people run NFS-mounted data directories ... -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc. ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
