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

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Message-ID: <alpine.DEB.2.00.0908251721170.28411@asgard.lang.hm>
Date:	Tue, 25 Aug 2009 17:26:11 -0700 (PDT)
From:	david@...g.hm
To:	Pavel Machek <pavel@....cz>
cc:	Ric Wheeler <rwheeler@...hat.com>, 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

On Wed, 26 Aug 2009, Pavel Machek wrote:

>>>>> ---
>>>>> 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.

if you are intentionally causing several low-probability things to happen 
at once you increase the risk of corruption

note that you also need a write to take place, and be interrupted in just 
the right way.

David Lang
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