Date: Tue, 22 Nov 2005 23:07:00 +0100 From: Tomas Zvala <tomas@zvala.cz> To: freebsd-geom@freebsd.org Subject: Re: A question on gmirror and "load sharing" Message-ID: <43839684.9050106@zvala.cz> In-Reply-To: <4383480A.7070606@withagen.nl> References: <4383480A.7070606@withagen.nl>
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Hello, I did some similar testing and came to similar conclusion. As PJD explained, the mirror will never double the transfer rate in sequential I/O. Here's the thread: http://docs.freebsd.org/cgi/getmsg.cgi?fetch=0+0+archive/2004/freebsd-geom/20041205.freebsd-geom Tomas btw. i never said thank you for his answer, so now i do :) thanks. Willem Jan Withagen wrote: > Hi, > > I'm in the proces of running some "performance" tests on raw disks, as > a small configuration test to run my NFS-perfomance tests on a > different setup. > > (http://withagen.dyndns.org/FreeBSD/Performance) > > So I build a mirror with gmirror and 2 80Gb WD-sata disks. > > On that I run tests like (for reading): > ---- > while ($i < 16000) > dd of=/dev/null if=/dev/ad4 bs=5M count=1 iseek=$i > end > ---- > > Giving me transfer times and rates that depend on the location of the > head on the platter. The graphs seem to suggest that this simple test > is not at all bad as an approach to this. > When you run this on an old BigFoot, which has a plexiglas view on the > heads. You really see the arm creeping slowly to the spindle. > > However when I run this read test on a mirrored set of disks. I see > (with systat -vm 1) that data is only transfered from 1 disk for a > long time. And then several seconds later the other disk is used. This > also happens if I create the mirror with 'gmirror label -b load'. And > as such the transferrate of the mirror is equal or less than a single > disk. > > Perhaps I should test this with 'split'? > Or is this something internal in the kernel that a 5Mb request comes > in, and is forwarded as a whole request down the filesystem. And thus > there is no parallellism to take advantage off. (Even on an SMP the > request is to one disk only) > > --WjW > _______________________________________________ > freebsd-geom@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-geom > To unsubscribe, send any mail to "freebsd-geom-unsubscribe@freebsd.org" >
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