Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

[Chris Mason <chris.mason@oracle.com>]

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.

-chris