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Date: Fri, 15 Jul 2005 21:29:28 +0200
From: Matthias Buelow <mkb@incubus.de>
To: John-Mark Gurney <gurney_j@resnet.uoregon.edu>
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Cc: freebsd-stable@freebsd.org
Subject: Re: dangerous situation with shutdown process
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John-Mark Gurney wrote:

>even request barries will not save the fs in a power loss if the track
>that is getting flushed durning a power loss...  Some other FreeBSD
>folk has a reproducable case of where blocks that were not written to
>on ATA hardware got trashed after a power loss...
>With non-written to sectors getting trashed with the cache enabled,
>barriers don't mean squat...

One more thought.. they _do_ protect against power loss during writing
a track -- when used in combination with a journalled fs.

A corrupted journal can be detected. If it's corrupted, discard
the whole thing, or only the relevant entry. The filesystem will
remain consistent.
If track corruption occurs after the journal is written, it doesn't
matter, since at boot the journal will be replayed and all operations
will be performed once more.

The combination barriers+journal really seems to be very resilient
to filesystem corruption. When it's implemented without errors, and
the hardware doesn't do things like change bits randomly, I can't
think of a way this scheme can be corrupted at all.

mkb.