linux-ext4 - Re: [patch] document flash/RAID dangers

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Message-ID: <alpine.DEB.2.00.0908251816300.30426@asgard.lang.hm>
Date:	Tue, 25 Aug 2009 18:17:02 -0700 (PDT)
From:	david@...g.hm
To:	Pavel Machek <pavel@....cz>
cc:	Theodore Tso <tytso@....edu>, Ric Wheeler <rwheeler@...hat.com>,
	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] document flash/RAID dangers

On Wed, 26 Aug 2009, Pavel Machek wrote:

> On Tue 2009-08-25 17:20:13, david@...g.hm wrote:
>> On Wed, 26 Aug 2009, Pavel Machek wrote:
>>
>>> On Tue 2009-08-25 16:56:40, david@...g.hm wrote:
>>>> On Wed, 26 Aug 2009, Pavel Machek wrote:
>>>>
>>>>> There are storage devices that high highly undesirable properties
>>>>> when they are disconnected or suffer power failures while writes are
>>>>> in progress; such devices include flash devices and MD RAID 4/5/6
>>>>> arrays.
>>>>
>>>> change this to say 'degraded MD RAID 4/5/6 arrays'
>>>>
>>>> also find out if DM RAID 4/5/6 arrays suffer the same problem (I strongly
>>>> suspect that they do)
>>>
>>> I changed it to say MD/DM.
>>>
>>>> then you need to add a note that if the array becomes degraded before a
>>>> scrub cycle happens previously hidden damage (that would have been
>>>> repaired by the scrub) can surface.
>>>
>>> I'd prefer not to talk about scrubing and such details here. Better
>>> leave warning here and point to MD documentation.
>>
>> I disagree with that, the way you are wording this makes it sound as if
>> raid isn't worth it. if you are going to say that raid is risky you need
>> to properly specify when it is risky
>
> Ok, would this help? I don't really want to go to scrubbing details.
>
> (*) Degraded array or single disk failure "near" the powerfail is
> neccessary for this property of RAID arrays to bite.

that sounds reasonable

David Lang
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