Date: Wed, 26 Jan 2005 17:42:50 -0800 (PST) From: Arne "Wörner" <arne_woerner@yahoo.com> To: David Schultz <das@FreeBSD.ORG> Cc: freebsd-fs@FreeBSD.ORG Subject: Re: ufs+softupdates / consistency Message-ID: <20050127014250.57722.qmail@web41204.mail.yahoo.com> In-Reply-To: <20050127004013.GA62561@VARK.MIT.EDU>
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--- David Schultz <das@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
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