On Sat, Aug 16, 2014 at 3:28 AM, Noah Misch <n...@leadboat.com> wrote:
> Nice algorithm.


>> I'd be afraid that a secondary mechanism that mostly-but-not-really
>> works could do more harm by allowing us to miss bugs in the primary,
>> pipe-based locking mechanism than the good it would accomplish.
> Users do corrupt their NFS- and GFS2-hosted databases today.  I would rather
> have each process hold only an fcntl() lock than hold only the FIFO file
> descriptor.  There's no such dichotomy, so let's have both.

Meh.  We can do that, but I think that will provide us with only the
it-works-until-it-doesn't level of protection.  Granted, that's more
than zero, but does anyone advocate wearing seatbelts for the first 60
minutes you're in the car and then taking them off after that?  I
think that with a sufficiently long-running server the chances of the
lock somehow getting released approach certainty.  But I'm not going
to fight this one tooth and nail.

A bigger question in my view is what to do with the existing
mechanism.  The main advantage of making a change like this is that we
could finally dispense with System V shared memory completely.  But we
risk encountering systems where the battle-tested System V mechanism
works and this new one either fails to work at all (server won't
start) or fails to work as desired (interlock broken).  So it's
tempting to think we should have a GUC or control-file setting to
control which mechanism gets used.  Of course for QNX, the actual
subject of this thread, System V won't be an option, but other people
might like a big red button they can push if the new code turns out to
be less than we're hoping.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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