linux-ext4 - Re: ext2/3: document conditions when reliable operation is possible

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Message-ID: <20090824092622.GC25591@elf.ucw.cz>
Date:	Mon, 24 Aug 2009 11:26:22 +0200
From:	Pavel Machek <pavel@....cz>
To:	Goswin von Brederlow <goswin-v-b@....de>
Cc:	Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	tytso@....edu, rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: ext2/3: document conditions when reliable operation is possible

Hi!

> >> > This is not about barriers (that should be different topic). Atomic
> >> > write means that either whole sector is written, or nothing at all is
> >> > written. Because raid5 needs to update both master data and parity at
> >> > the same time, I don't think it can guarantee this during powerfail.
> 
> Actualy raid5 should have no problem with a power failure during
> normal operations of the raid. The parity block should get marked out
> of sync, then the new data block should be written, then the new
> parity block and then the parity block should be flaged in sync.
> 
> >> Good point, but I thought that's what journaling was for?
> >
> > I believe journaling operates on assumption that "either whole sector
> > is written, or nothing at all is written".
> 
> The real problem comes in degraded mode. In that case the data block
> (if present) and parity block must be written at the same time
> atomically. If the system crashes after writing one but before writing
> the other then the data block on the missng drive changes its
> contents. And for example with a chunk size of 1MB and 16 disks that
> could be 15MB away from the block you actualy do change. And you can
> not recover that after a crash as you need both the original and
> changed contents of the block.
> 
> So writing one sector has the risk of corrupting another (for the FS)
> totally unconnected sector. No amount of journaling will help
> there. The raid5 would need to do journaling or use battery backed
> cache.

Thanks, I updated my notes.
									Pavel
-- 
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