linux-kernel - Re: [RFC PATCH] fpathconf() for fsync() behavior

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Date:	Thu, 23 Apr 2009 18:23:53 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Valerie Aurora Henson <vaurora@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Chris Mason <chris.mason@...cle.com>,
	Theodore Tso <tytso@....edu>,
	Eric Sandeen <sandeen@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>
Subject: Re: [RFC PATCH] fpathconf() for fsync() behavior

Valerie Aurora Henson wrote:
> All that being said, I'd be thrilled to have fine-grained fsync().

Me too.

Ted raises a very good point that fine-grained fsync will not be used
by most applications, and they expect good behaviour automatically on
crashes without it (which is imho reasonable to ask for).

A lot of apps and scripts go wrong if the disk is full too.  I've seen
more truncated files from that than from system crashes.

I think both events are so rare that most of the time nobody cares.
They're corner cases.  Let's face it, nearly every shell script which
edits files in a specific order (see also "make") will see
inconsistencies following a system crashes.

But the thing is: certain core packages where reliability is a
requirement will use whatever mechanisms are available.  Every mail
transport and database engine seems to get this right - or try their
best given limitations of the OS - it's their job to care.  Those are
widely used by other apps.

Let's face it, like most other authors, if powerfail-robustness were
that important to us on linux-kernel, barriers would never have been
off by default on ext3, and fsync would always have emitted barriers.

The thing is: you _can_ expect certain core packages, used by a large
number of apps, to make use of whatever features work well.  Make
those reliable and you've solved a big chunk of the problem.  Make the
core packages able to perform well at the same time, and a few more
apps will use them instead of their own implementations.

-- Jamie
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