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

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Message-ID: <20090826004430.GR4300@elf.ucw.cz>
Date:	Wed, 26 Aug 2009 02:44:31 +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


>>>> 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, no S-ATA drive has capacitors to hold up during a power failure 
> (or even enough power to destage their write cache). I know this from 
> direct, personal knowledge having built RAID boxes at EMC for years. In 
> fact, almost all RAID boxes require that the write cache be hardwired to 
> off when used in their arrays.

I never claimed they have enough power to flush entire cache -- read
the paragraph again. I do believe the disks have enough capacitors to
finish writing single sector, and I do 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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