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:	Sun, 30 Aug 2009 18:44:04 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	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)

Ric Wheeler wrote:
[]
> The easiest way to lose your data in Linux - with RAID, without RAID, 
> S-ATA or SAS - is to run with the write cache enabled.
> 
> If you compare the size of even a large RAID stripe it will be measured 
> in KB and as this thread has mentioned already, you stand to have damage 
> to just one stripe (or even just a disk sector or two).
> 
> 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.

> 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.

/mjt
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