From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gionatan Danti <g.danti@assyoma.it>
Subject: Re: Filesystem corruption on RAID1
Date: Fri, 18 Aug 2017 21:42:25 +0200
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To: Roger Heflin <rogerheflin@gmail.com>
Cc: Wols Lists <antlists@youngman.org.uk>, Reindl Harald <h.reindl@thelounge.net>, Roman Mamedov <rm@romanrm.net>, Linux RAID <linux-raid@vger.kernel.org>
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Il 18-08-2017 14:54 Roger Heflin ha scritto:
> I have noticed all of the hardware raid controllers explicitly turn
> off the disk's write cache so this would eliminate this issue, but the
> cost is much slower write times.

True...

> It makes the hardware raid controllers (and disk arrays) become
> uselessly slow when their battery backup dies and disables the raid
> card and/or arrays write cache.

...true...

> Remember, safe, fast and cheap, you only get to pick 2.   We generally
> pick fast and cheap, the disk arrays/raid controllers pick safe and
> fast, but not so cheap as a hardware raid controller with write cache
> backup of some sort are quite expensive.

...and true. I am not arguing any of these points.

What really surprised me was to realize that, facing micro-powerlosses, 
*even sync() writes* can be vulnerable to undetected data loss, at least 
when not using FUAs (using instead the common barrier infrastructure).

So while many (old) mismatch_cnt reports on RAID1/10 arrays where 
dismissed as "don't bother, it's a harmless RAID1 thing", I really think 
than some were genuine corruptions due to micro powerlosses and similar 
causes.

If nothing more, such reports really emphasize the need to have a 
"trusted" mismatch_cnt for mirrored arrays, even in the face of some 
performance losses (due to no using zero copy anymore).

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
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