Date: Mon, 3 Apr 2017 11:09:08 -0700
From: Jeremy Allison <jra@samba.org>
To: Jeff Layton <jlayton@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>, NeilBrown <neilb@suse.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-ext4@vger.kernel.org, akpm@linux-foundation.org, tytso@mit.edu,
        jack@suse.cz
Subject: Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking
 infrastructure and convert ext4 to use it
Message-ID: <20170403180908.GG37923@jra3>
Reply-To: Jeremy Allison <jra@samba.org>
References: <20170331192603.16442-1-jlayton@redhat.com>
 <87fuhqkti0.fsf@notabene.neil.brown.name>
 <1491215318.2724.3.camel@redhat.com>
 <20170403143257.GA30811@bombadil.infradead.org>
 <1491241657.2673.10.camel@redhat.com>
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On Mon, Apr 03, 2017 at 01:47:37PM -0400, Jeff Layton wrote:
> On Mon, 2017-04-03 at 07:32 -0700, Matthew Wilcox wrote:
> > On Mon, Apr 03, 2017 at 06:28:38AM -0400, Jeff Layton wrote:
> > > On Mon, 2017-04-03 at 14:25 +1000, NeilBrown wrote:
> > > > Also I think that EIO should always over-ride ENOSPC as the possible
> > > > responses are different.  That probably means you need a separate seq
> > > > number for each, which isn't ideal.
> > > > 
> > > 
> > > I'm not quite convinced that it's really useful to do anything but
> > > report the latest error.
> > > 
> > > But...if we did need to prefer one over another, could we get away with
> > > always reporting -EIO once that error occurs? If so, then we'd still
> > > just need a single sequence counter.
> > 
> > I wonder whether it's even worth supporting both EIO and ENOSPC for a
> > writeback problem.  If I understand correctly, at the time of write(),
> > filesystems check to see if they have enough blocks to satisfy the
> > request, so ENOSPC only comes up in the writeback context for thinly
> > provisioned devices.
> > 
> > Programs have basically no use for the distinction.  In either case,
> > the situation is the same.  The written data is safely in RAM and cannot
> > be written to the storage.  If one were to make superhuman efforts,
> > one could mmap the file and write() it to a different device, but that
> > is incredibly rare.  For most programs, the response is to just die and
> > let the human deal with the corrupted file.
> > 
> > From a sysadmin point of view, of course the situation is different,
> > and the remedy is different, but they should be getting that information
> > through a different mechanism than monitoring the errno from every
> > system call.
> > 
> > If we do want to continue to support both EIO and ENOSPC from writeback,
> > then let's have EIO override ENOSPC as an error.  ie if an ENOSPC comes
> > in after an EIO is set, it only bumps the counter and applications will
> > see EIO, not ENOSPC on fresh calls to fsync().
> 
> 
> No, ENOSPC on writeback can certainly happen with network filesystems.
> NFS and CIFS have no way to reserve space. You wouldn't want to have to
> do an extra RPC on every buffered write. :)

CIFS has a way to reserve space. Look into "allocation size" on create.