linux-kernel - Re: POSIX violation by writeback error

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Message-ID: <20180927142738.GA27040@thunk.org>
Date:   Thu, 27 Sep 2018 10:27:38 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        焦晓冬 <milestonejxd@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rogier Wolff <R.E.Wolff@...Wizard.nl>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: POSIX violation by writeback error

On Thu, Sep 27, 2018 at 08:43:10AM -0400, Jeff Layton wrote:
> 
> Basically, the problem (as I see it) is that we can end up evicting
> uncleanable data from the cache before you have a chance to call fsync,
> and that means that the results of a read after a write are not
> completely reliable.

Part of the problem is that people don't agree on what the problem is.  :-)

The original posting was from someone who claimed it was a "POSIX
violation" if a subsequent read returns *successfully*, but then the
writeback succeeds.

Other people are worried about this problem; yet others are worried
about the system wedging and OOM-killing itself, etc.

The problem is that in the face of I/O errors, it's impossible to keep
everyone happy.  (You could make the local storage device completely
reliable, with a multi-million dollar storage array with remote
replication, but then the CFO won't be happy; and other people were
talking about making things work with cheap USB thumb drives and
laptops.  This is the very definition of an over-constained problem.)

       	      	   	      	    		    - Ted