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

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Message-ID: <20090826000657.GK4300@elf.ucw.cz>
Date:	Wed, 26 Aug 2009 02:06:57 +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

On Tue 2009-08-25 19:48:09, Ric Wheeler 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.

> 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).

>> Otherwise, file systems placed on these devices can suffer silent data
>> and file system corruption.  An forced use of fsck may detect metadata
>> corruption resulting in file system corruption, but will not suffice
>> to detect data corruption.
>>
>
> This is very misleading. All storage "can" have silent data loss, you are 
> making a statement without specifics about frequency.

substitute with "can (by design)"?

Now, if you can suggest useful version of that document meeting your
criteria?

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