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Date: Thu, 30 Oct 2008 20:41:59 -0500
From: Rich Winkel <rich@math.missouri.edu>
To: Jeremy Chadwick <koitsu@FreeBSD.org>
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Cc: Rich Winkel <rich@math.missouri.edu>, freebsd-questions@FreeBSD.org
Subject: Re: Filesystem, RAID Question
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On Thu, Oct 30, 2008 at 04:38:49PM -0700, Jeremy Chadwick wrote:
> On Thu, Oct 30, 2008 at 06:12:07PM -0500, Rich Winkel wrote:
> > Doesn't hw.ata.wc affect only card-level caching?
> 
> hw.ata.wc causes the ata(4) subsystem to disable write caching on all
> disks attached to the subsystem.  It does not disable card features.

I mean, the individual disks are invisible to the OS unless the
card's driver (and the card itself) specifically supports it.

> There's also the below PR, which extends atacontrol to permit disabling
> and enabling write caching on a per-disk basis.
> 
> http://www.freebsd.org/cgi/query-pr.cgi?pr=127717

But not on disks which are behind hardware raid cards, correct?

> What gives you the impression that during a power outage your data is
> going to be intact?

One of the main functions of softupdates is to order disk updates in such
a way that the fs organizational integrity is maintained at all times.
Of course this doesn't protect against actual sector corruption, but if
the disk is between writes at the time it loses power, the fs structure
is supposed to still be internally consistent.  At least that's my
understanding of it.

Rich