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Re: Posix Shared Mem patch

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "A(dot)M(dot)" <agentm(at)themactionfaction(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Posix Shared Mem patch
Date: 2012-06-27 14:06:56
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Wed, Jun 27, 2012 at 3:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
>> On Wed, Jun 27, 2012 at 3:50 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I wonder whether this design can be adapted to Windows?  IIRC we do
>>> not have a bulletproof data directory lock scheme for Windows.
>>> It seems like this makes few enough demands on the lock mechanism
>>> that there ought to be suitable primitives available there too.
>> I assume you're saying we need to make changes in the internal API,
>> right? Because we alreayd have a windows native implementation of
>> shared memory that AFAIK works,
> Right, but does it provide honest protection against starting two
> postmasters in the same data directory?  Or more to the point,
> does it prevent starting a new postmaster when the old postmaster
> crashed but there are still orphaned backends making changes?
> AFAIR we basically punted on those problems for the Windows port,
> for lack of an equivalent to nattch.

No, we spent a lot of time trying to *fix* it, and IIRC we did.

We create a shared memory segment with a fixed name based on the data
directory. This shared memory segment is inherited by all children. It
will automatically go away only when all processes that have an open
handle to it go away (in fact, it can even take a second or two more,
if they go away by crash and not by cleanup - we have a workaround in
the code for that). But as long as there is an orphaned backend
around, the shared memory segment stays around.

We don't have "nattch". But we do have "nattch>0". Or something like that.

You can work around it if you find two different paths to the same
data directory (e.g .using junctions), but you are really actively
trying to break the system if you do that...

 Magnus Hagander

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