Date: Wed, 2 Sep 2009 22:12:10 +0200
From: Pavel Machek <pavel@ucw.cz>
To: Ric Wheeler <rwheeler@redhat.com>
Cc: david@lang.hm, Theodore Tso <tytso@mit.edu>,
       Florian Weimer <fweimer@bfk.de>,
       Goswin von Brederlow <goswin-v-b@web.de>, Rob Landley <rob@landley.net>,
       kernel list <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, mtk.manpages@gmail.com,
       rdunlap@xenotime.net, linux-doc@vger.kernel.org,
       linux-ext4@vger.kernel.org, corbet@lwn.net
Subject: Re: [testcase] test your fs/storage stack (was Re: [patch] ext2/3:
	document conditions when reliable operation is possible)
Message-ID: <20090902201210.GC1840@ucw.cz>
References: <20090826001645.GN4300@elf.ucw.cz> <4A948259.40007@redhat.com> <20090826010018.GA17684@mit.edu> <4A948C94.7040103@redhat.com> <20090826025849.GF32712@mit.edu> <4A9510D2.1090704@redhat.com> <20090826111208.GA26595@elf.ucw.cz> <alpine.DEB.2.00.0908260416270.30426@asgard.lang.hm> <20090829094919.GF1634@ucw.cz> <4A9910D5.4060208@redhat.com>
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>>> people aren't objecting to better documentation, they are objecting to
>>> misleading documentation.
>>>      
>> Actually Ric is. He's trying hard to make RAID5 look better than it
>> really is.
>
> I object to misleading and dangerous documentation that you have  
> proposed. I spend a lot of time working in data integrity, talking and  
> writing about it so I care deeply that we don't misinform people.

Yes, truth is dangerous. To vendors selling crap products. 

> In this thread, I put out a draft that is accurate several times and you  
> have failed to respond to it.

Accurate as in 'has 0 information content' :-(.

> The big picture that you don't agree with is:
>
> (1) RAID (specifically MD RAID) will dramatically improve data integrity  
> for real users. This is not a statement of opinion, this is a statement  
> of fact that has been shown to be true in large scale deployments with  
> commodity hardware.

It is also completely irrelevant.

> (2) RAID5 protects you against a single failure and your test case  
> purposely injects a double failure.

Most people would be surprised that press of reset button is 'failure'
in this context.

> (4) Data loss occurs in non-journalling file systems and journalling  
> file systems when you suffer double failures or hot unplug storage,  
> especially inexpensive FLASH parts.

It does not happen on inexpensive DISK parts,  so people do not expect
that and it is worth pointing out.
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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