linux-ext4 - Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

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Message-ID: <1491241657.2673.10.camel@redhat.com>
Date:   Mon, 03 Apr 2017 13:47:37 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     NeilBrown <neilb@...e.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
        akpm@...ux-foundation.org, tytso@....edu, jack@...e.cz
Subject: Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking
 infrastructure and convert ext4 to use it

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. :)
-- 
Jeff Layton <jlayton@...hat.com>