linux-ext4 - Re: [patch] ext2/3: document conditions when reliable operation is possible

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Date:	Mon, 24 Aug 2009 16:22:22 -0600
From:	Zan Lynx <zlynx@....org>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Pavel Machek <pavel@....cz>, 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
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
 possible

Ric Wheeler wrote:
> Pavel Machek wrote:
>> Degraded MD RAID5 does not work by design; whole stripe will be
>> damaged on powerfail or reset or kernel bug, and ext3 can not cope
>> with that kind of damage. [I don't see why statistics should be
>> neccessary for that; the same way we don't need statistics to see that
>> ext2 needs fsck after powerfail.]
>>                                     Pavel
>>   
> What you are describing is a double failure and RAID5 is not double 
> failure tolerant regardless of the file system type....

Are you sure he isn't talking about how RAID must write all the data 
chunks to make a complete stripe and if there is a power-loss, some of 
the chunks may be written and some may not?

As I read Pavel's point he is saying that the incomplete write can be 
detected by the incorrect parity chunk, but degraded RAID-5 has no 
working parity chunk so the incomplete write would go undetected.

I know this is a RAID failure mode. However, I actually thought this was 
a problem even for a intact RAID-5. AFAIK, RAID-5 does not generally 
read the complete stripe and perform verification unless that is 
requested, because doing so would hurt performance and lose the entire 
point of the RAID-5 rotating parity blocks.

-- 
Zan Lynx
zlynx@....org

"Knowledge is Power.  Power Corrupts.  Study Hard.  Be Evil."
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