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

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Date:	Tue, 25 Aug 2009 16:03:18 -0700 (PDT)
From:	david@...g.hm
To:	Pavel Machek <pavel@....cz>
cc:	Ric Wheeler <rwheeler@...hat.com>, 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 Wed, 26 Aug 2009, Pavel Machek wrote:

>>>> I don't object to making that general statement - "Don't hot unplug a
>>>> device with an active file system or actively used raw device" - but
>>>> would object to the overly general statement about ext3 not working on
>>>> flash, RAID5 not working, etc...
>>>
>>> You can object any way you want, but running ext3 on flash or MD RAID5
>>> is stupid:
>>>
>>> * ext2 would be faster
>>>
>>> * ext2 would provide better protection against powerfail.
>>
>> Not true in the slightest, you continue to ignore the ext2/3/4 developers
>> telling you that it will lose data.
>
> I know I will lose data. Both ext2 and ext3 will lose data on
> flashdisk. (That's what I'm trying to document). But... what is the
> benefit of ext3 journaling on MD RAID5? (On flash, ext3 at least
> protects you against kernel panic. MD RAID5 is in software, so... that
> additional protection is just not there).

the block device can loose data, it has absolutly nothing to do with the 
filesystem

>>> "ext3 works on flash and MD RAID5, as long as you do not have
>>> powerfail" seems to be the accurate statement, and if you don't need
>>> to protect against powerfails, you can just use ext2.
>>
>> Strange how your personal preference is totally out of sync with the
>> entire enterprise class user base.
>
> Perhaps noone told them MD RAID5 is dangerous? You see, that's exactly
> what I'm trying to document here.

a MD raid array that's degraded to the point where there is no redundancy 
is dangerous, but I don't think that any of the enterprise users would be 
surprised.

I think they will be surprised that it's possible that a prior failed 
write that hasn't been scrubbed can cause data loss when the array later 
degrades.

David Lang
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