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

> On Sun, Apr 10, 2011 at 5:03 PM, A.M. <age...@themactionfaction.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.

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