linux-kernel - Re: Grace period

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Date:	Mon, 9 Apr 2012 16:33:36 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"bfields@...ldses.org" <bfields@...ldses.org>
CC:	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	Jeff Layton <jlayton@...hat.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Grace period

On Mon, 2012-04-09 at 12:21 -0400, bfields@...ldses.org wrote:
> On Mon, Apr 09, 2012 at 04:17:06PM +0000, Myklebust, Trond wrote:
> > On Mon, 2012-04-09 at 12:11 -0400, bfields@...ldses.org wrote:
> > > On Mon, Apr 09, 2012 at 08:08:57PM +0400, Stanislav Kinsbursky wrote:
> > > > 09.04.2012 19:27, Jeff Layton пишет:
> > > > >
> > > > >If you allow one container to hand out conflicting locks while another
> > > > >container is allowing reclaims, then you can end up with some very
> > > > >difficult to debug silent data corruption. That's the worst possible
> > > > >outcome, IMO. We really need to actively keep people from shooting
> > > > >themselves in the foot here.
> > > > >
> > > > >One possibility might be to only allow filesystems to be exported from
> > > > >a single container at a time (and allow that to be overridable somehow
> > > > >once we have a working active/active serving solution). With that, you
> > > > >may be able limp along with a per-container grace period handling
> > > > >scheme like you're proposing.
> > > > >
> > > > 
> > > > Ok then. Keeping people from shooting themselves here sounds reasonable.
> > > > And I like the idea of exporting a filesystem only from once per
> > > > network namespace.
> > > 
> > > Unfortunately that's not going to get us very far, especially not in the
> > > v4 case where we've got the common read-only pseudoroot that everyone
> > > has to share.
> > 
> > I don't see how that can work in cases where each container has its own
> > private mount namespace. You're going to have to tie that pseudoroot to
> > the mount namespace somehow.
> 
> Sure, but in typical cases it'll still be shared; requiring that they
> not be sounds like a severe limitation.

I'd expect the typical case to be the non-shared namespace: the whole
point of containers is to provide for complete isolation of processes.
Usually that implies that you don't want them to be able to communicate
via a shared filesystem.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com