linux-ext4 - Re: [patch] document flash/RAID dangers

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Message-ID: <20090826002045.GO4300@elf.ucw.cz>
Date:	Wed, 26 Aug 2009 02:20:45 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	david@...g.hm, Theodore Tso <tytso@....edu>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	Rob Landley <rob@...dley.net>,
	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: [patch] document flash/RAID dangers

>>>> ---
>>>> There are storage devices that high highly undesirable properties
>>>> when they are disconnected or suffer power failures while writes are
>>>> in progress; such devices include flash devices and MD RAID 4/5/6
>>>> arrays.  These devices have the property of potentially
>>>> corrupting blocks being written at the time of the power failure, and
>>>> worse yet, amplifying the region where blocks are corrupted such that
>>>> additional sectors are also damaged during the power failure.
>>>
>>> I would strike the entire mention of MD devices since it is your
>>> assertion, not a proven fact. You will cause more data loss from common
>>
>> That actually is a fact. That's how MD RAID 5 is designed. And btw
>> those are originaly Ted's words.
>
> Ted did not design MD RAID5.

So what? He clearly knows how it works.

Instead of arguing he's wrong, will you simply label everything as
unproven?

>>> events (single sector errors, complete drive failure) by steering people
>>> away from more reliable storage configurations because of a really rare
>>> edge case (power failure during split write to two raid members while
>>> doing a RAID rebuild).
>>
>> I'm not sure what's rare about power failures. Unlike single sector
>> errors, my machine actually has a button that produces exactly that
>> event. Running degraded raid5 arrays for extended periods may be
>> slightly unusual configuration, but I suspect people should just do
>> that for testing. (And from the discussion, people seem to think that
>> degraded raid5 is equivalent to raid0).
>
> Power failures after a full drive failure with a split write during a rebuild?

Look, I don't need full drive failure for this to happen. I can just
remove one disk from array. I don't need power failure, I can just
press the power button. I don't even need to rebuild anything, I can
just write to degraded array.

Given that all events are under my control, statistics make little
sense here.
								Pavel

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