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

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Date:	Fri, 3 Apr 2009 09:33:49 +0200
From:	Pavel Machek <pavel@....cz>
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,
	Matthew Garrett <mjg59@...f.ucam.org>,
	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"


> If you are a mail client developer, and the user says, "I want the
> advantages of IMAP, but I refuse to switch to an ISP that provides
> IMAP; you must give me *all* the advantages IMAP even though I'm using
> POP3", you'd probably tell the user, "Yes, and do you want a pony,
> too?"

Somebody wants a pony?

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

>From what I seen on the gtk lists, application developers are willing
to change they code. _But_ we should make sure that it does not
regress. fsync() is a regression: spins the disk up too much, slow on
ext3. (They may be willing to do that, but I believe that's a very bad
idea). And yes, I hope your "lets add fsync() everywhere, then break
the fsync with eat-my-data-^W-laptop-mode" plan does not
happen. (Please acknowledge that it is a stupid idea...)

If you give them fbarrier() or replace() or something that is nop or
nearly so on ext3 data=ordered and fixes ext4/btrfs, they'll happily
use it. But we do not have such thing now, and we should not be really
asking them to regress on existing setups.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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