linux-kernel - Re: Ext4 and the "30 second window of death"

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Message-ID: <20090403000039.GD9538@srcf.ucam.org>
Date:	Fri, 3 Apr 2009 01:00:39 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Theodore Tso <tytso@....edu>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Ray Lee <ray-lk@...rabbit.org>, david@...g.hm,
	Sitsofe Wheeler <sitsofe@...oo.com>,
	Alberto Gonzalez <info@...bu.es>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Ext4 and the "30 second window of death"

On Thu, Apr 02, 2009 at 07:38:06PM -0400, Theodore Tso wrote:

> What's been frustrating about this whole controversy is this implicit
> assumptions that users and applications should never change, and the
> filesystem should magically accomodate and Do The Right Thing.

This is the attitude that I have a significant problem with. Filesystems 
exist to serve applications. Without applications, there's no reason to 
have a filesystem. If a filesystem doesn't provide the behaviour that 
applications want then that filesystem has no reason to exist. The aim 
isn't to produce a platonically ideal filesystem. The aim is to produce 
a filesystem that behaves well given the applications that use it.

Disagreeing with the behaviour of applications is a perfectly sensible 
thing to do. However, it's something that should be done at the *start* 
of a filesystem development cycle. Getting agreement from a broad 
section of application developers means that you get to write a 
filesystem that embodies a different set of assumptions and everyone 
wins. Writing a filesystem and then bitching about application behaviour 
after it's been merged to mainline is just pathological.

> The problem is, this is what the application programmers are telling
> the filesystem developers.  They refuse to change their programs; and
> the features they want are sometimes mutually contradictory, or at
> least result in a overconstrained problem --- and then they throw the
> whole mess at the filesystem developers' feet and say, "you fix it!"

Which application developers did you speak to? Because, frankly, the 
majority of the ones I know felt that ext3 embodied the pony that they'd 
always dreamed of as a five year old. Stephen gave them that pony almost 
a decade ago and now you're trying to take it to the glue factory. I 
remember almost crying at that bit on Animal Farm, so I'm really not 
surprised that you're getting pushback here.

> I'm not saying the filesystems are blameless, but give us a little
> slack, guys; we NEED some help from the application developers here.

Then having a discussion with application developers over the 
expectations they can have would be a good first step. Just pointing at 
POSIX isn't good enough - POSIX allows a bunch of behaviours 
sufficiently pathological that a filesystem implementing them would be 
less useful than /dev/null. We need to have a worthwhile conversation 
about what guarantees Linux will provide above and beyond POSIX. The 
filesystem summit next week isn't going to be that conversation. Perhaps 
something to try at Plumbers?

-- 
Matthew Garrett | mjg59@...f.ucam.org
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