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Date:	Tue, 25 Aug 2009 14:08:10 -0700 (PDT)
From:	david@...g.hm
To:	Rob Landley <rob@...dley.net>
cc:	Greg Freemyer <greg.freemyer@...il.com>,
	Pavel Machek <pavel@....cz>, Ric Wheeler <rwheeler@...hat.com>,
	Theodore Tso <tytso@....edu>, Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	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
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
 possible

On Tue, 25 Aug 2009, Rob Landley wrote:

> On Monday 24 August 2009 16:11:56 Greg Freemyer wrote:
>>> The papers show failures in "once a year" range. I have "twice a
>>> minute" failure scenario with flashdisks.
>>>
>>> Not sure how often "degraded raid5 breaks ext3 atomicity" would bite,
>>> but I bet it would be on "once a day" scale.
>>
>> I agree it should be documented, but the ext3 atomicity issue is only
>> an issue on unexpected shutdown while the array is degraded.  I surely
>> hope most people running raid5 are not seeing that level of unexpected
>> shutdown, let along in a degraded array,
>>
>> If they are, the atomicity issue pretty strongly says they should not
>> be using raid5 in that environment.  At least not for any filesystem I
>> know.  Having writes to LBA n corrupt LBA n+128 as an example is
>> pretty hard to design around from a fs perspective.
>
> Right now, people think that a degraded raid 5 is equivalent to raid 0.  As
> this thread demonstrates, in the power failure case it's _worse_, due to write
> granularity being larger than the filesystem sector size.  (Just like flash.)
>
> Knowing that, some people might choose to suspend writes to their raid until
> it's finished recovery.  Perhaps they'll set up a system where a degraded raid
> 5 gets remounted read only until recovery completes, and then writes go to a
> new blank hot spare disk using all that volume snapshoting or unionfs stuff
> people have been working on.  (The big boys already have hot spare disks
> standing by on a lot of these systems, ready to power up and go without human
> intervention.  Needing two for actual reliability isn't that big a deal.)
>
> Or maybe the raid guys might want to tweak the recovery logic so it's not
> entirely linear, but instead prioritizes dirty pages over clean ones.  So if
> somebody dirties a page halfway through a degraded raid 5, skip ahead to
> recover that chunk first to the new disk first (yes leaving holes, it's not that
> hard to track), and _then_ let the write go through.
>
> But unless people know the issue exists, they won't even start thinking about
> ways to address it.

if you've got the drives available you should be running raid 6 not raid 5 
so that you have to loose two drives before you loose your error checking.

in my opinion that's a far better use of a drive than a hot spare.

David Lang
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