ufs+softupdates / consistency

[David Schultz]

On Wed, Jan 26, 2005, Arne WXrner wrote:
> I would be glad, if somebody explains me, why ext2fs/async in
> Linux kernel 2.4.27 (KNOPPIX V3.7) is much faster (about 4 times
> faster) than a ufs with soft updates on the same slice of the hard
> disc?
> 
> Is it due to consistency reasons? In case of a ext2fs/sync in my
> Linux setting Linux was about 4 times slower.

ext2fs mounted async does not provide consistency; in fact the
state of the disk can be almost arbitrarily inconsistent at any
given time.  Soft updates is supposed to provide performance
comparable to async writes without the inconsistency problem.  I'm
not sure what it is about your setup that causes such a disparity.
(Many factors such as the FS block size and ATA write caching can
make a big difference.)

By the way, ext3fs uses journalling, which provides metadata
consistency in a very different way from soft updates.  You might
also want to experiment with that to see if it works better for
your workload.

> Are we already trying to issue write order requests for the disc
> blocks (whose write order is arbitrary) sorted by sector number
> (in order to move the disc heads as less as possible)? The disc
> write cache could do that, but I disabled it in order to decrease
> the probability of inconsistency.

Hopefully you disabled it on both FreeBSD and Linux, so you're
comparing apples to apples...