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

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Message-ID: <20090826001206.GL4300@elf.ucw.cz>
Date:	Wed, 26 Aug 2009 02:12:06 +0200
From:	Pavel Machek <pavel@....cz>
To:	david@...g.hm
Cc:	Theodore Tso <tytso@....edu>, Ric Wheeler <rwheeler@...hat.com>,
	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 16:56:40, david@...g.hm wrote:
> 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.
>
> change this to say 'degraded MD RAID 4/5/6 arrays'
>
> also find out if DM RAID 4/5/6 arrays suffer the same problem (I strongly 
> suspect that they do)

I changed it to say MD/DM.

> then you need to add a note that if the array becomes degraded before a  
> scrub cycle happens previously hidden damage (that would have been  
> repaired by the scrub) can surface.

I'd prefer not to talk about scrubing and such details here. Better
leave warning here and point to MD documentation.

>> THESE devices have the property of potentially corrupting blocks being  
>> written at the time of the power failure,
>
> this is true of all devices

Actually I don't think so. I believe SATA disks do not corrupt even
the sector they are writing to -- they just have big enough
capacitors. And yes I believe ext3 depends on that.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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