david@xxxxxxx escribió: > On Thu, 28 Aug 2008, Scott Marlowe wrote: >> scenario 1: There's a postmaster, it owns all the child processes. >> It gets killed. The Postmaster gets restarted. Since there isn't one > > when the postmaster gets killed doesn't that kill all it's children as > well? Of course not. The postmaster gets a SIGKILL, which is instant death. There's no way to signal the children. If they were killed too then this wouldn't be much of a problem. >> running, it comes up. starts new child processes. Meanwhile, the old >> child processes that don't belong to it are busy writing to the data >> store. Instant corruption. > > if so then the postmaster should not only check if there is an existing > postmaster running, it should check for the presense of the child > processes as well. See my other followup. There's limited things it can check, but against sysadmin stupidity there's no silver bullet. > well, if you aren't going through the postmaster, what process is > recieving network messages? it can't be a group of processes, only one > can be listening to a socket at one time. Huh? Each backend has its own socket. > and if the postmaster isn't needed for the child processes to write to > the datastore, how are multiple child processes prevented from writing to > the datastore normally? and why doesn't that mechanism continue to work? They use locks. Those locks are implemented using shared memory. If a new postmaster starts, it gets a new shared memory, and a new set of locks, that do not conflict with the ones already held by the first gang of backends. This is what causes the corruption. > so are you saying that the only possible thing that can kill the > postmaster is the OOM killer? it can't possilby exit in any other > situation without the children being shutdown first? > > I would be surprised if that was really true. If the sysadmin sends a SIGKILL then obviously the same thing happens. Any other signal gives it the chance to signal the children before dying. -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc.
