linux-ext4 - fsck more often when powerfail is detected (was Re: wishful thinking about atomic, multi-sector or full MD stripe width, writes in storage)

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Date:	Sun, 4 Apr 2010 15:47:29 +0200
From:	Pavel Machek <pavel@....cz>
To:	Theodore Tso <tytso@....edu>, Ric Wheeler <rwheeler@...hat.com>,
	Krzysztof Halasa <khc@...waw.pl>,
	Christoph Hellwig <hch@...radead.org>, Mark Lord <lkml@....ca>,
	Michael Tokarev <mjt@....msk.ru>, david@...g.hm,
	NeilBrown <neilb@...e.de>, Rob Landley <rob@...dley.net>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: fsck more often when powerfail is detected (was Re: wishful
	thinking about atomic, multi-sector or full MD stripe width, writes
	in storage)

Hi!

> > Yes, but ext3 was designed to handle the partial write  (according to
> > tytso).
> 
> I'm not sure what made you think that I said that.  In practice things
> usually work out, as a conseuqence of the fact that ext3 uses physical
> block journaling, but it's not perfect, becase...

Ok; so the journalling actually  is not reliable on many machines --
not even disk drive manufacturers guarantee full block writes AFAICT.

Maybe there's time to reviwe the patch to increase mount count by >1
when journal is replayed, to do fsck more often when powerfails are
present?


> > > Also, when you enable the write cache (MD or not) you are buffering 
> > > multiple MB's of data that can go away on power loss. Far greater (10x) 
> > > the exposure that the partial RAID rewrite case worries about.
> > 
> > Yes, that's what barriers are for. Except that they are not there on
> > MD0/MD5/MD6. They actually work on local sata drives...
> 
> Yes, but ext3 does not enable barriers by default (the patch has been
> submitted but akpm has balked because he doesn't like the performance
> degredation and doesn't believe that Chris Mason's "workload of doom"
> is a common case).  Note though that it is possible for dirty blocks
> to remain in the track buffer for *minutes* without being written to
> spinning rust platters without a barrier.

So we do wrong thing by default. Another reason to do fsck more often
when powerfails are present?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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