Re: File Systems Compared

On Wed, Dec 06, 2006 at 08:55:14 -0800,
Mark Lewis <mark(dot)lewis(at)mir3(dot)com> wrote:
> > Anyone run their RAIDs with disk caches enabled, or is this akin to
> > having fsync off?
>
> Disk write caches are basically always akin to having fsync off. The
> only time a write-cache is (more or less) safe to enable is when it is
> backed by a battery or in some other way made non-volatile.
>
> So a RAID controller with a battery-backed write cache can enable its
> own write cache, but can't safely enable the write-caches on the disk
> drives it manages.

This appears to be changing under Linux. Recent kernels have write barriers
implemented using cache flush commands (which some drives ignore, so you
need to be careful). In very recent kernels, software raid using raid 1
will also handle write barriers. To get this feature, you are supposed to
mount ext3 file systems with the barrier=1 option. For other file systems,
the parameter may need to be different.