> On Tue, 15 Jan 2008 20:24:27 -0500
> "Daniel Phillips" <phillips@google.com> wrote:
>
> > On Jan 15, 2008 7:15 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> > > > Writeback cache on disk in iteself is not bad, it only gets bad
> > > > if the disk is not engineered to save all its dirty cache on
> > > > power loss, using the disk motor as a generator or alternatively
> > > > a small battery. It would be awfully nice to know which brands
> > > > fail here, if any, because writeback cache is a big performance
> > > > booster.
> > >
> > > AFAIK no drive saves the cache. The worst case cache flush for
> > > drives is several seconds with no retries and a couple of minutes
> > > if something really bad happens.
> > >
> > > This is why the kernel has some knowledge of barriers and uses them
> > > to issue flushes when needed.
> >
> > Indeed, you are right, which is supported by actual measurements:
> >
> >
http://sr5tech.com/write_back_cache_experiments.htm
> >
> > Sorry for implying that anybody has engineered a drive that can do
> > such a nice thing with writeback cache.
> >
> > The "disk motor as a generator" tale may not be purely folklore. When
> > an IDE drive is not in writeback mode, something special needs to done
> > to ensure the last write to media is not a scribble.
> >
> > A small UPS can make writeback mode actually reliable, provided the
> > system is smart enough to take the drives out of writeback mode when
> > the line power is off.
>
> We've had mount -o barrier=1 for ext3 for a while now, it makes
> writeback caching safe. XFS has this on by default, as does reiserfs.
