linux-ext4 - Re: [testcase] test your fs/storage stack (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Message-ID: <20090905102810.GA1341@ucw.cz>
Date:	Sat, 5 Sep 2009 12:28:10 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Rob Landley <rob@...dley.net>, jim owens <jowens@...com>,
	david@...g.hm, 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, corbet@....net
Subject: Re: [testcase] test your fs/storage stack (was Re: [patch] ext2/3:
	document conditions when reliable operation is possible)

On Fri 2009-09-04 07:49:34, Ric Wheeler wrote:
> On 09/04/2009 03:44 AM, Rob Landley wrote:
>> On Thursday 03 September 2009 09:14:43 jim owens wrote:
>>    
>>> Rob Landley wrote:
>>>      
>>>> I think he understands he was clueless too, that's why he investigated
>>>> the failure and wrote it up for posterity.
>>>>
>>>>        
>>>>> And Ric said do not stigmatize whole classes of A) devices, B) raid,
>>>>> and C) filesystems with "Pavel says...".
>>>>>          
>>>> I don't care what "Pavel says", so you can leave the ad hominem at the
>>>> door, thanks.
>>>>        
>>> See, this is exactly the problem we have with all the proposed
>>> documentation.  The reader (you) did not get what the writer (me)
>>> was trying to say.  That does not say either of us was wrong in
>>> what we thought was meant, simply that we did not communicate.
>>>      
>> That's why I've mostly stopped bothering with this thread.  I could respond to
>> Ric Wheeler's latest (what does write barriers have to do with whether or not
>> a multi-sector stripe is guaranteed to be atomically updated during a panic or
>> power failure?) but there's just no point.
>>    
>
> The point of that post was that the failure that you and Pavel both  
> attribute to RAID and journalled fs happens whenever the storage cannot  
> promise to do atomic writes of a logical FS block (prevent torn  
> pages/split writes/etc). I gave a specific example of why this happens  
> even with simple, single disk systems.

ext3 does not expect atomic write of 4K block, according to Ted. So
no, it is not broken on single disk.
 
>> The LWN article on the topic is out, and incomplete as it is I expect it's the
>> best documentation anybody will actually _read_.

Would anyone (probably privately?) share the lwn link?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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