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

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Date:	Wed, 26 Aug 2009 08:01:53 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	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: [patch] ext2/3: document conditions when reliable operation is
 possible

On 08/26/2009 07:12 AM, Pavel Machek wrote:
> On Wed 2009-08-26 06:39:14, Ric Wheeler wrote:
>    
>> On 08/25/2009 10:58 PM, Theodore Tso wrote:
>>      
>>> On Tue, Aug 25, 2009 at 09:15:00PM -0400, Ric Wheeler wrote:
>>>
>>>        
>>>> I agree with the whole write up outside of the above - degraded RAID
>>>> does meet this requirement unless you have a second (or third, counting
>>>> the split write) failure during the rebuild.
>>>>
>>>>          
>>> The argument is that if the degraded RAID array is running in this
>>> state for a long time, and the power fails while the software RAID is
>>> in the middle of writing out a stripe, such that the stripe isn't
>>> completely written out, we could lose all of the data in that stripe.
>>>
>>> In other words, a power failure in the middle of writing out a stripe
>>> in a degraded RAID array counts as a second failure.
>>>     To me, this isn't a particularly interesting or newsworthy point,
>>> since a competent system administrator who cares about his data and/or
>>> his hardware will (a) have a UPS, and (b) be running with a hot spare
>>> and/or will imediately replace a failed drive in a RAID array.
>>>        
>> I agree that this is not an interesting (or likely) scenario, certainly
>> when compared to the much more frequent failures that RAID will protect
>> against which is why I object to the document as Pavel suggested. It
>> will steer people away from using RAID and directly increase their
>> chances of losing their data if they use just a single disk.
>>      
> 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?
> 									Pavel
>    

I am against documenting unlikely scenarios out of context that will 
lead people to do the wrong thing.

ric


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