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Date: Fri, 17 May 2002 18:35:37 -0700
From: Mike Fedyk <mfedyk@matchmail.com>
To: "Stephen C. Tweedie" <sct@redhat.com>
Cc: Neil Brown <neilb@cse.unsw.edu.au>, linux-kernel@vger.kernel.org
Subject: Re: Thoughts on using fs/jbd from drivers/md
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On Fri, May 17, 2002 at 07:34:10PM +0100, Stephen C. Tweedie wrote:
> Hi,
> 
> On Fri, May 17, 2002 at 11:29:42AM -0700, Mike Fedyk wrote:
> > On Thu, May 16, 2002 at 04:17:49PM +0100, Stephen C. Tweedie wrote:
> 
> > > Right.  The ability of soft raid5 to lose data in degraded mode over a
> > > reboot (including data that was not being modified at the time of the
> > > crash) is something that is not nearly as widely understood as it
> > > should be, and I'd love for us to do something about it.
> > 
> > Are there workarounds to avoid this problem?
> 
> No.
> 
> > What does it take to trigger the corruption?
> 
> It just takes degraded mode, an unexpected power cycle, and concurrent
> write activity.  
> 
> Degraded mode relies on the parity disk being in sync at all times ---

Doesn't degraded mode imply that there are not any parity
disk(raid4)/stripe(raid5) updates?

> you can't recover data from the missing spindle unless that is true.
> However, writes to a stripe are not atomic, and you can get a reboot
> when, say, a write to one of the surviving data chunks has succeeded,
> but the corresponding write to the parity disk has not.  If this
> happens, the parity is no longer in sync, and the data belonging to
> the missing spindle in that stripe will be lost forever.
> 
> > I ask this because I have used a degraded raid5 because the source drive
> > would become a member, but I needed to copy the data first.  While doing so,
> > I had to reboot a couple times to reconfigure the boot loader.  All seems to
> > be working fine on the system today though.
> 
> If it was a clean shutdown and reboot, you're fine.
>

OK, that's good.