A question on gmirror and "load sharing"

[Tomas Zvala]

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
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