linux-ext4 - Re: fsync() errors is unsafe and risks data loss

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Date:   Fri, 13 Apr 2018 06:25:35 -0700
From:   Andres Freund <andres@...razel.de>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Dave Chinner <david@...morbit.com>,
        lsf-pc <lsf-pc@...ts.linuxfoundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andreas Dilger <adilger@...ger.ca>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "Joshua D. Drake" <jd@...mandprompt.com>
Subject: Re: fsync() errors is unsafe and risks data loss

Hi,

On 2018-04-13 09:18:56 -0400, Jeff Layton wrote:
> Yes, I think we ought to probably do the same thing globally. It's nice
> to know that xfs has already been doing this. That makes me feel better
> about making this behavior the gold standard for Linux filesystems.
> 
> So to summarize, at this point in the discussion, I think we want to
> consider doing the following:
> 
> * better reporting from syncfs (report an error when even one inode
> failed to be written back since last syncfs call). We'll probably
> implement this via a per-sb errseq_t in some fashion, though there are
> some implementation issues to work out.
> 
> * invalidate or clear uptodate flag on pages that experience writeback
> errors, across filesystems. Encourage this as standard behavior for
> filesystems and maybe add helpers to make it easier to do this.
> 
> Did I miss anything? Would that be enough to help the Pg usecase?
> 
> I don't see us ever being able to reasonably support its current
> expectation that writeback errors will be seen on fd's that were opened
> after the error occurred. That's a really thorny problem from an object
> lifetime perspective.

It's not perfect, but I think the amount of hacky OS specific code
should be acceptable. And it does allow for a wrapper tool that can be
used around backup restores etc to syncfs all the necessary filesystems.
Let me mull with others for a bit.

Greetings,

Andres Freund