linux-ext4 - Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Message-ID: <4A9BCEA8.9080206@redhat.com>
Date:	Mon, 31 Aug 2009 09:22:48 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Michael Tokarev <mjt@....msk.ru>, david@...g.hm,
	Pavel Machek <pavel@....cz>, Theodore Tso <tytso@....edu>,
	NeilBrown <neilb@...e.de>, Rob Landley <rob@...dley.net>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	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: raid is dangerous but that's secret (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)

On 08/31/2009 09:16 AM, Christoph Hellwig wrote:
> On Mon, Aug 31, 2009 at 09:15:27AM -0400, Ric Wheeler wrote:
>>> While most common filesystem do have barrier support it is:
>>>
>>>    - not actually enabled for the two most common filesystems
>>>    - the support for write barriers an cache flushing tends to be buggy
>>>      all over our software stack,
>>>
>>
>> Or just missing - I think that MD5/6 simply drop the requests at present.
>>
>> I wonder if it would be worth having MD probe for write cache enabled&
>> warn if barriers are not supported?
>
> In my opinion even that is too weak.  We know how to control the cache
> settings on all common disks (that is scsi and ata), so we should always
> disable the write cache unless we know that the whole stack (filesystem,
> raid, volume managers) supports barriers.  And even then we should make
> sure the filesystems does actually use barriers everywhere that's needed
> which failed at for years.
>

I was thinking about that as well. Having us disable the write cache when we 
know it is not supported (like in the MD5 case) would certainly be *much* safer 
for almost everyone.

We would need to have a way to override the write cache disabling for people who 
either know that they have a non-volatile write cache (unlikely as it would 
probably be to put MD5 on top of a hardware RAID/external array, but some of the 
new SSD's claim to have non-volatile write cache).

It would also be very useful to have all of our top tier file systems enable 
barriers by default, provide consistent barrier on/off mount options and log a 
nice warning when not enabled....

ric

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