> 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.
