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Re: We should Axe /contrib/start-scripts

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Chander Ganesan <chander(at)otg-nc(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: We should Axe /contrib/start-scripts
Date: 2009-08-27 06:54:22
Message-ID: 9837222c0908262354i4b7e45c4t51f81c2b284265dc@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Aug 27, 2009 at 02:38, Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I did have another thought. It could compare the time from uptime to
>> the timestamp on the lock file. If the server's been restarted since
>> the time in the lock file then it must be stale. uhm. unless clock's
>> been changed...
>
> Yeah, you can't trust system clocks too much either :-(.
>
> I was actually having second thoughts about the idea of using file
> locking.  The only environment in which I've heard of file locks not
> being trustworthy is NFS, and if you're running a DB on NFS you've
> probably got worse problems than this one.

That is a bad generalization. A lot of people run their databases very
successfully on NFS. It just requires that you have a good NFS server,
know how to set it up, know how to set up your network, have a good
NFS client and know how to set *it* up.

Though I would assume that locks would be trustworthy in this case as well...


> Notably, if you mistakenly
> try to start postmasters on two different machines against the same
> NFS-mounted directory, the PID-based interlock will certainly fail, while
> file locking might save you.

That's in no way limited to NFS though... The difference being that in
a lot of other cases you just end up with a completely corrupt
filesystem :)


>  So maybe we should take another look at
> that.  Has anyone heard of other contexts in which file locks don't
> work?  Has Windows got them?

Certainly: http://msdn.microsoft.com/en-us/library/aa365202%28VS.85%29.aspx
for example.


-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

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