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Re: initdb dies during IpcSemaphoreCreate under BSD jail

From: Alex Hayward <xelah(at)xelah(dot)com>
To: PostgresSQL Hackers Mailing List <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: initdb dies during IpcSemaphoreCreate under BSD jail
Date: 2002-03-27 10:56:24
Message-ID: Pine.LNX.4.33.0203271047050.6431-100000@sphinx.mythic-beasts.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, 27 Mar 2002, Tom Lane wrote:

> "Joel Burton" <joel(at)joelburton(dot)com> writes:
> >> This will allow you to run a single postgres in a single jail only one
> >> user would have access to it.  If you try to run more then one it will
> >> try to use the same shared memory and crash.
>
> > Is this, in fact, the case?
>
> Unless BSD jails have very bizarre shared memory behavior, this is
> nonsense.  PG can easily run multiple postmasters in the same machine
> (there are currently four postmasters of different vintages alive on
> the machine I'm typing this on).  Give each one a different database
> directory and a unique port number, and you're good to go.
>
> It might be that postmasters in different jails on the same machine
> would have to be assigned different port numbers to keep them from
> conflicting.  Don't know exactly how airtight a BSD jail is ...
> but there is an interaction between port number and shared memory
> key.  I can imagine that a jail that hides processes but not shared
> memory segments might confuse our startup logic that tries to detect
> whether an existing shared memory segment is safe to reuse or not.
> Perhaps your ISP has seen failures of that type from trying to
> start multiple postmasters on the same port number in different
> jails.

FreeBSD jails are supposed to put just about everything in to different
namespaces/contention domains/whatever. You can't see processes running
outside a jail from within it, you can't see files outside your jail, you
can only use your jail's IP address, etc. However, this doesn't work for
SYSV IPC (not in FreeBSD-STABLE, at least) and everything goes in to one
machine-wide namespace - hence the sysctl to turn it on/off.

PostgreSQL will run quite happily using different port numbers in
different jails - but the port numbers MUST be different. Since the ISP is
probably using jails to make multiple users as unaware of each other as
possible this might be a problem for them...

You should probably also consider that someone in /another/ jail might be
able to get access to your shared memory segments. This would, most
likely, be a bad thing to happen.


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