Re: POSIX shared memory redux

On Apr 11, 2011, at 6:06 PM, Robert Haas wrote:

> On Sun, Apr 10, 2011 at 5:03 PM, A.M. <agentm(at)themactionfaction(dot)com> wrote:
>> To ensure that no two postmasters can startup in the same data directory, I use fcntl range locking on the data directory lock file, which also works properly on (properly configured) NFS volumes. Whenever a postmaster or postmaster child starts, it acquires a read (non-exclusive) lock on the data directory's lock file. When a new postmaster starts, it queries if anything would block a write (exclusive) lock on the lock file which returns a lock-holding PID in the case when other postgresql processes are running.
>
> This seems a lot leakier than what we do now (imagine, for example,
> shared storage) and I'm not sure what the advantage is. I was
> imagining keeping some portion of the data in sysv shm, and moving the
> big stuff to a POSIX shm that would operate alongside it.

What do you mean by "leakier"? The goal here is to extinguish SysV shared memory for portability and convenience benefits. The mini-SysV proposal was implemented and shot down by Tom Lane.

Cheers,
M