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

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Message-ID: <alpine.DEB.2.00.0908310844230.6822@asgard.lang.hm>
Date:	Mon, 31 Aug 2009 08:45:38 -0700 (PDT)
From:	david@...g.hm
To:	Pavel Machek <pavel@....cz>
cc:	George Spelvin <linux@...izon.com>, 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:

On Mon, 31 Aug 2009, Pavel Machek wrote:

>> 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.

note that all that zfs does is tell you that you already lost data (and 
then only if the checksumming algorithm would be invalid on a blank block 
being returned), it doesn't protect your data.

David Lang
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