GEOM mirror performance - slow

[Pawel Jakub Dawidek]

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.
[...]

Hard to say, really. It depends on many factors. On my test system:

	# dd if=/dev/zero of=/dev/ad4d bs=1m count=1000 &; dd if=/dev/zero of=/dev/ad6d bs=1m count=1000
	ad4d:		64.27MB/s
	ad6d:		61.49MB/s

	# gmirror label foo ad4d ad6d
	# dd if=/dev/zero of=/dev/mirror/foo bs=1m count=1000
	mirror/foo:	61.26MB/s

BUT disks can somehow interfere with each other (are on the same ATA
channel, etc.).

On this system I've this setup:

ATA channel 2:
    Master:  ad4 <ST3808110AS/3.AAD> Serial ATA II
    Slave:   ad5 <ST3808110AS/3.AAD> Serial ATA II
ATA channel 3:
    Master:  ad6 <ST3808110AS/3.AAD> Serial ATA II
    Slave:       no device present

As you saw above, I was using ad4 and ad6, which are on different ATA
channels. When I do the same experiment for ad4 and ad5 I see worser
results:

	# dd if=/dev/zero of=/dev/ad4d bs=1m count=1000 &; dd if=/dev/zero of=/dev/ad5d bs=1m count=1000
	ad4d:		59.55MB/s
	ad5d:		59.31MB/s

	# gmirror label foo ad4d ad5d
	# dd if=/dev/zero of=/dev/mirror/foo bs=1m count=1000
	mirror/foo:	49.03MB/s

So not ~40% slowdown as in your case, but still visible slowdown (~20%).

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