ufs+softupdates / consistency

[Arne Wörner]

--- David Schultz <das at FreeBSD.ORG> wrote:
> On Wed, Jan 26, 2005, Arne WXrner wrote:
> > > > 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...
> > > 
> > During the tests I enabled write cache in both settings,
> > because I did not know how to turn it off in KNOPPIX...
> > 
> > Isn't it possible to simulate the hard disc write cache in
> > kernel?
> 
> The write caching will make a big difference.  Sorry, but I
> don't know how to turn it off under Linux.  Perhaps you could
> turn it on under FreeBSD for the purpose of your performance
> analysis.
> 
Yes, I did. I enabled it in both test settings (KNOPPIX and
FreeBSD). But I set hw.ata.wc to 0 in my every day setting.

I just tried "dd of=a if=/dev/zero bs=32k count=1000" on a file
system with hard disc write cache enabled and disabled and I saw
no difference (appr. 5 Mbyte/sec, which is about 5 times less than
a read from that file after the kernel cache was clear by other
reads).

Can somebody explain me, why hw.ata.wc does not change write
speed?
I would prefer hw.ata.wc=0, because it would be part of a damage
avoidance system (I do not use an UPS and I have some write
accesses over quite long periods of time caused by my TV cards
(sys/dev/bktr)).

Can somebody explain me, why write speed is so much slower than
read speed (even with hard disc write cache)?

I tried an UFS1 file system mounted async for another test. And
the write speed was still about 5 Mbyte/sec.

Can somebody explain me, what async filesystem I/O is (somehow my
english is not sufficient to find that out)?

-Arne



		
__________________________________ 
Do you Yahoo!? 
Take Yahoo! Mail with you! Get it on your mobile phone. 
http://mobile.yahoo.com/maildemo