linux-ext4 - Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:

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Message-ID: <20090831105645.GD1353@ucw.cz>
Date:	Mon, 31 Aug 2009 13:04:48 +0200
From:	Pavel Machek <pavel@....cz>
To:	George Spelvin <linux@...izon.com>
Cc:	linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:

Hi!

> Actually, there is something the file system can do to make journaling
> safe on degraded RAIDs: make the (checksummed) journal blocks equal to
> the RAID stripe size.  Or, equivalently, pad out to the RAID stripe
> size each commit.
> 
> This sometimes leads to awkward block sizes, but while writing
> to any *one* stripe on a degraded RAID-5 endangers the others, you
> can write to *all* of them with the usual semantics.

Well, that would work... but you'd also have to journal data, with the
same block size. Not exactly fast, but at least safe...

> That's one thing I really like about ZFS: its policy of "don't trust
> the disks."  If nothing else, simply telling you "your disks f*ed up,
> and I caught them doing it", instead of the usual mysterious corruption
> detectec three months later, is tremendoudly useful information.

The more I learn about storage, the more I like idea of zfs. Given the
subtle issues between filesystem and raid layer, integrating them just
makes sense.
								Pavel

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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