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@themactionfaction.com> wrote:
>> To ensure that no two postmasters can startup in the same data directory, I use fcntl range locking on the data
directorylock file, which also works properly on (properly configured) NFS volumes. Whenever a postmaster or postmaster
childstarts, it acquires a read (non-exclusive) lock on the data directory's lock file. When a new postmaster starts,
itqueries if anything would block a write (exclusive) lock on the lock file which returns a lock-holding PID in the
casewhen 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