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

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Date:	Sat, 29 Aug 2009 07:28:21 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	david@...g.hm, 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: [testcase] test your fs/storage stack (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)

On 08/29/2009 05:49 AM, Pavel Machek wrote:
>    
>>> So instead of fixing or at least documenting known software deficiency
>>> in Linux MD stack, you'll try to surpress that information so that
>>> people use more of raid5 setups?
>>>
>>> Perhaps the better documentation will push them to RAID1, or maybe
>>> make them buy an UPS?
>>>        
>> 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.

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

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.

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

(3) How to configure MD reliably should be documented in MD 
documentation, not in each possible FS or raw device application

(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.

ric

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