On Apr 13, 2011, at 2:06 AM, Tom Lane wrote:

> "A.M." <age...@themactionfaction.com> writes:
>> On Apr 11, 2011, at 7:13 PM, Tom Lane wrote:
>>> Robert Haas <robertmh...@gmail.com> writes:
>>>> I mean I'm not convinced that fcntl() locking will be as reliable.
>>> I'm not either.  Particularly not on NFS.
>> Is there an example of a recent system where fcntl is broken (ignoring NFS)?
> Well, the fundamental point is that "ignoring NFS" is not the real
> world.  We can't tell people not to put data directories on NFS,
> and even if we did tell them not to, they'd still do it.  And NFS
> locking is not trustworthy, because the remote lock daemon can crash
> and restart (forgetting everything it ever knew) while your own machine
> and the postmaster remain blissfully awake.
> None of this is to say that an fcntl lock might not be a useful addition
> to what we do already.  It is to say that fcntl can't just replace what
> we do already, because there are real-world failure cases that the
> current solution handles and fcntl alone wouldn't.

The goal of this patch is to eliminate SysV shared memory, not to implement 
NFS-capable locking which, as you point out, is virtually impossible.

As far as I can tell, in the worst case, my patch does not change how 
postgresql handles the NFS case. SysV shared memory won't work across NFS, so 
that interlock won't catch, so postgresql is left with looking at a lock file 
with PID of process on another machine, so that won't catch either. This patch 
does not alter the lock file semantics, but merely augments the file with file 

At least with this patch, there is a chance the lock might work across NFS. In 
the best case, it can allow for shared-storage postgresql failover, which is a 
new feature.

Furthermore, there is an improvement in shared memory handling in that it is 
unlinked immediately after creation, so only the postmaster and its children 
have access to it (through file descriptor inheritance). This means shared 
memory cannot be stomped on by any other process.

Considering that possibly working NFS locking is a side-effect of this patch 
and not its goal and, in the worst possible scenario, it doesn't change current 
behavior, I don't see how this can be a ding against this patch.

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