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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Date:	Mon, 31 Aug 2009 09:15:27 -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 08/30/2009 12:35 PM, Christoph Hellwig wrote:
> On Sun, Aug 30, 2009 at 06:44:04PM +0400, Michael Tokarev wrote:
>>> If you lose power with the write caches enabled on that same 5 drive
>>> RAID set, you could lose as much as 5 * 32MB of freshly written data on
>>>   a power loss (16-32MB write caches are common on s-ata disks these
>>> days).
>>
>> This is fundamentally wrong.  Many filesystems today use either barriers
>> or flushes (if barriers are not supported), and the times when disk drives
>> were lying to the OS that the cache got flushed are long gone.
>
> 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?

>>> For MD5 (and MD6), you really must run with the write cache disabled
>>> until we get barriers to work for those configurations.
>>
>> I highly doubt barriers will ever be supported on anything but simple
>> raid1, because it's impossible to guarantee ordering across multiple
>> drives.  Well, it *is* possible to have write barriers with journalled
>> (and/or with battery-backed-cache) raid[456].
>>
>> Note that even if raid[456] does not support barriers, write cache
>> flushes still works.
>
> All currently working barrier implementations on Linux are built upon
> queue drains and cache flushes, plus sometimes setting the FUA bit.
>

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