On Tue, 15 Jan 2008 20:24:27 -0500 "Daniel Phillips" <phillips@xxxxxxxxxx> wrote: > On Jan 15, 2008 7:15 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> 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 - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
