Disabling write-behind on IDE drives, and SMART

[Erik Trulsson]

On Fri, Jan 21, 2005 at 08:31:06AM -0800, Kevin Oberman wrote:
> > Date: Fri, 21 Jan 2005 20:53:12 +1300
> > From: Mark Kirkwood <markir at paradise.net.nz>
> > Sender: owner-freebsd-stable at freebsd.org
> > 
> > Jim C. Nasby wrote:
> > > Question one: how do I disable write caching on IDE drives? I know the
> > > setting is hw.ata.wc=0, but where do I put that? In loader.conf? This is
> > > FreeBSD 4.10, btw.
> > >
> > In /boot/loader.conf will do it.
> > 
> > > Also, has any thought been given to making the default 0, like it is for
> > > SCSI devices? I'm honestly surprised and disappointed that the default
> > > is speed over data integrity.
> > I think it was trialled in 4.3 then changed for 4.4 (according to the 
> > handbook, the performance hit was considered to be too high)
> 
> Having been involved in the 4.x flip-flop, the performance cost on some
> systems can be HUGE! Backing up my laptop (40 GB) went from 40 minutes
> to five hours when the write cache was turned off. 
> 
> On the other hand, it is a real risk, especially when combined with
> softupdates.

I don't see how softupdates can increase the risk. As I understand it
the risks with IDE-write chaching is just as large with softupdates as
without.  It is just that all the safety and consistency guarantees
that softupdates normally make go out of the window when write-caching
is turned on.

> For servers that lack solid backup power (not a 10 minute
> UPS), I would probably turn it off. But for most systems it is probably
> worth the risks.

A 10 minute UPS should be plenty for that purpose. All that is needed
to protect against the risks of write-caching is for the server to be
able to make an orderly shutdown, which should not take more than a
couple of minutes.  (If you need constant uptime, you need better power
backup, but that is a different issue.)



-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at student.uu.se