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: <20090830163513.GA25899@infradead.org>
Date:	Sun, 30 Aug 2009 12:35:13 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Ric Wheeler <rwheeler@...hat.com>, 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 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,

>> 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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