linux-ext4 - Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Message-ID: <alpine.DEB.2.00.0908300550320.6822@asgard.lang.hm>
Date:	Sun, 30 Aug 2009 05:55:01 -0700 (PDT)
From:	david@...g.hm
To:	Pavel Machek <pavel@....cz>
cc:	Theodore Tso <tytso@....edu>, NeilBrown <neilb@...e.de>,
	Ric Wheeler <rwheeler@...hat.com>,
	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: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)

On Sun, 30 Aug 2009, Pavel Machek wrote:

>>> From: Theodore Tso <tytso@....edu>
>>>
>> To use your ABS brakes analogy, just becase it's not safe to rely on
>> ABS brakes if the "check brakes" light is on, that doesn't justify
>> writing something alarmist which claims that ABS brakes don't work
>> 100% of the time, don't use ABS brakes, they're broken!!!!
>
> If it only was this simple. We don't have 'check brakes' (aka
> 'journalling ineffective') warning light. If we had that, I would not
> have problem.
>
> It is rather that your ABS brakes are ineffective if 'check engine'
> (RAID degraded) is lit. And yes, running with 'check engine' for
> extended periods may be bad idea, but I know people that do
> that... and I still hope their brakes work (and believe they should
> have won suit for damages should their ABS brakes fail).

the 'RAID degraded' warning says that _anything_ you put on that block 
device is at risk. it doesn't matter if you are using a filesystem with a 
journal, one without, or using the raw device directly.

David Lang
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