dangerous situation with shutdown process

[David Sze]

On Thu, Jul 14, 2005 at 09:52:53PM +0200, Matthias Buelow wrote:
> Kevin Oberman wrote:
> 
> >The problem is that disks lie about whether they have actually written
> >data. If the power goes off before the data is in cache, it's lost.
> 
> No, the problem is that FreeBSD doesn't implement request barriers
> and that softupdates is flawed by design and seemingly could not
> make use of them, even if they were available (because, as I
> understand it, it relies on a total ordering of all writes, unlike
> the partial ordering necessary for a journalled fs).
> 
> Until a journalled fs that uses write request barriers is available
> for FreeBSD, you better had a reliable UPS.

How do OS-level request barriers help if the disk reorders pending
writes in its cache?