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

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Message-ID: <20180412054536.GI729@dastard>
Date:   Thu, 12 Apr 2018 15:45:36 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Andres Freund <andres@...razel.de>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        20180410184356.GD3563@...nk.org,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Jeff Layton <jlayton@...hat.com>,
        "Joshua D. Drake" <jd@...mandprompt.com>
Subject: Re: fsync() errors is unsafe and risks data loss

On Wed, Apr 11, 2018 at 07:32:21PM -0700, Andres Freund wrote:
> Hi,
> 
> On 2018-04-12 10:09:16 +1000, Dave Chinner wrote:
> > To pound the broken record: there are many good reasons why Linux
> > filesystem developers have said "you should use direct IO" to the PG
> > devs each time we have this "the kernel doesn't do <complex things
> > PG needs>" discussion.
> 
> I personally am on board with doing that. But you also gotta recognize
> that an efficient DIO usage is a metric ton of work, and you need a
> large amount of differing logic for different platforms. It's just not
> realistic to do so for every platform.  Postgres is developed by a small
> number of people, isn't VC backed etc. The amount of resources we can
> throw at something is fairly limited.  I'm hoping to work on adding
> linux DIO support to pg, but I'm sure as hell not going to do be able to
> do the same on windows (solaris, hpux, aix, ...) etc.
> 
> And there's cases where that just doesn't help at all. Being able to
> untar a database from backup / archive / timetravel / whatnot, and then
> fsyncing the directory tree to make sure it's actually safe, is really
> not an insane idea.

Yes it is. 

This is what syncfs() is for - making sure a large amount of of data
and metadata spread across many files and subdirectories in a single
filesystem is pushed to stable storage in the most efficient manner
possible.

> Or even just cp -r ing it, and then starting up a
> copy of the database.  What you're saying is that none of that is doable
> in a safe way, unless you use special-case DIO using tooling for the
> whole operation (or at least tools that fsync carefully without ever
> closing a fd, which certainly isn't the case for cp et al).

No, Just saying fsyncing individual files and directories is about
the most inefficient way you could possible go about doing this.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com