ZFS I/O Throughput question..

[Pawel Jakub Dawidek]

On Fri, Sep 17, 2010 at 09:35:21PM +0200, Pawel Jakub Dawidek wrote:
> On Wed, Sep 15, 2010 at 03:05:46AM -0500, Chris Watson wrote:
> > I have been testing ZFS on a home box now for a few days and I have a  
> > question that is perplexing me. Everything I have read on ZFS says in  
> > almost every case mirroring is faster than raidz. So I initially setup  
> > a 2x2 Raid 10 striped mirror. Like so:
> [...]
> 
> Could you try running something like this:
> 
> 	# apply "dd if=/dev/ada%1 of=/dev/null bs=1m count=5000 &" 2 3 4 5
> 
> This will tell us how much of total throughput do you have.
> If you can destroy your data, you may also try this:
> 
> 	# apply "dd if=/dev/null of=/dev/ada%1 bs=1m count=5000 &" 2 3 4 5

 	# apply "dd if=/dev/zero of=/dev/ada%1 bs=1m count=5000 &" 2 3 4 5

Thanks to se@ for noticing this.

> If you disks cannot work at full speed in parallel this might explain
> what you're seeing. Mirror send to disk twice as much data as it
> receives and RAIDZ sends only 33% more data in four disk case.
> 
> And no, there are neither special RAIDZ optimizations not special mirror
> pesimizations in FreeBSD.

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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