linux-kernel - Re: POSIX violation by writeback error

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Message-ID: <49555682.rh9yPTYizh@merkaba>
Date:   Wed, 05 Sep 2018 11:32:32 +0200
From:   Martin Steigerwald <martin@...htvoll.de>
To:     Rogier Wolff <R.E.Wolff@...wizard.nl>
Cc:     Jeff Layton <jlayton@...hat.com>,
        焦晓冬 <milestonejxd@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: POSIX violation by writeback error

Rogier Wolff - 05.09.18, 10:04:
> On Wed, Sep 05, 2018 at 09:39:58AM +0200, Martin Steigerwald wrote:
> > Rogier Wolff - 05.09.18, 09:08:
> > > So when a mail queuer puts mail the mailq files and the mail
> > > processor can get them out of there intact, nobody is going to
> > > notice.  (I know mail queuers should call fsync and report errors
> > > when that fails, but there are bound to be applications where
> > > calling fsync is not appropriate (*))
> > 
> > AFAIK at least Postfix MDA only reports mail as being accepted over
> > SMTP once fsync() on the mail file completed successfully. And I´d
> > expect every sensible MDA to do this. I don´t know how Dovecot MDA
> > which I currently use for sieve support does this tough.
> 
> Yes. That's why I added the remark that mailers will call fsync and
> know about it on the write side. I encountered a situation in the
> last few days that when a developer runs into this while developing,
> would have caused him to write:
>   /* Calling this fsync causes unacceptable performance */
>   // fsync (fd);

Hey, I still have

# KDE Sync
# Re: zero size file after power failure with kernel 2.6.30.5
# http://permalink.gmane.org/gmane.comp.file-systems.xfs.general/30512
export KDE_EXTRA_FSYNC=1

in my ~/.zshrc.

One reason KDE developers did this was Ext3 having been so slow with 
fsync(). See also:

Bug 187172 - truncated configuration files on power loss or hard crash 
https://bugs.kde.org/187172

> But when apt-get upgrade replaces your /bin/sh and gets a write error
> returning error on subsequent reads is really bad.

I sometimes used eatmydata with apt upgrade / dist-upgrade, but yeah, 
this asks for trouble on write interruptions.
 
> It is more difficult than you think.

Heh. :)

Thanks,
-- 
Martin