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

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Message-ID: <alpine.DEB.2.00.0908251817390.30426@asgard.lang.hm>
Date:	Tue, 25 Aug 2009 18:19:40 -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:

>>>>> 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.

keep in mind that in a powerfail situation the data being sent to the 
drive may be corrupt (the ram gets flaky while a DMA to the drive copies 
the bad data to the drive, which writes it before the power loss gets bad 
enough for the drive to decide there is a problem and shutdown)

you just plain cannot count on writes that are in flight when a powerfail 
happens to do predictable things, let alone what you consider sane or 
proper.

David Lang
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