GEOM mirror performance - slow

[R. B. Riddick]

--- Pawel Jakub Dawidek <pjd at FreeBSD.org> wrote:
> On Mon, Jan 08, 2007 at 10:57:14PM -0500, Ahnjoan Amous wrote:
> > I'm sorry about the double posting, I have switch e-mail clients and
> > hope that the text is now understandable.
> > 
> > While deploying 6.1-RELEASE I'm seeing slower than expected speed
> > using geom mirror.
> [...]
> 
> So not ~40% slowdown as in your case, but still visible slowdown (~20%).
> 
I have a theory now (when I read it first I was completely puzzled about this):
(aa) Since gmirror issues the write requests nearly at the same time, and (bb)
since gmirror has to wait for the completion of BOTH requests, and (cc) since
the two dd processes are not coupled, (dd) we see here the effects of
synchronization and bus/controller contention.

In your case (Ahnjoan) this effect is worse (lower DMA mode? older controller?)
than in yours (Pawel).

Thanks.

-Arne

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