Re: Understanding bonnie++ results

Hi,

OK. How fast are the Fujitsu disks, measured by a simple  hdparm -t?

Timing buffered disk reads:  270 MB in  3.02 seconds =  89.52 MB/sec

OK, so the problem you reported earlier, that HW raid was faster than
raid10,f2 for writing, is gone?

Well actually not for sequetial output. Here are the most comparable
figures:

4dk hw raid 10, 256k chunks versus 4dks md raid 10, 256k chunks
per char sequential output = 69475 vs 69928   <------ this is similar
block sequential output = 159649 vs 93985     <------ hw is much faster
rewrite sequential output = 85914 vs 56930    <------ hw is much faster

But on reading, md is faster:
per char sequential input = 61622 vs 68669    <------ still comparable
block sequential input = 221771 vs 356923     <------ md is way faster
ransom seek = 1327.1 vs 1149.7                <------ that's a 15.4%
difference

And you do a HW RAID10? Are you able to specify chunk size here?

Yes, RAID10 is an option in Adaptec's bios, and chink size can be set up
to 512k

quoted

-sequential input varies greatly, the big winner being md-f2-256 setup
with 356923K/sec, and the big loser md-near-64 setup with 34888K/sec
(factor of 10 !)

Both the chunk size, and the observation that raid10,n2 only reads from one
disk at a time, gives reasons to this. I already explained why raid10,f2
would be faster than HW RAID10.

True, and quite impressive.

quoted

- what seems the most relevant to me, random seeks are always better on
software raid, by 10 to 20%, but I have no idea why.

raid10.f2 woul only seek on half the disk, so that would diminish the
seek times.

Great. But in fact md raid 10 near layout (with 64k chunks, that might
matter), gave me slitghly better results than l2 (1347.3 for near versus
1327.1 for far)

quoted

- and running two bonnie++ in parallel on two 4 disks arrays gives
better iops than 6 disks arrays.

I would run a combined 12 disk array raid10,f2 with adequate chunk size,
I think that would get the best performance for you.

I will try that.

quoted

So I tend to think I'd better use md-f2-256 with 3 arrays of 4 disks and
use tablespaces to make sure my requests are spread out on the 3 arrays.
But this conclusion may suffer from many many flaws, the first one being
my understanding of raid, fs and io :)

So, any comment ?

I would try to test it out, but I don't know if you can get a good
benchmark for database enquieries.

That's the real problem for sure. I can throw in some huge queries, but
kernel resources and postgresql.conf clearly will change things much
more than raw disk io. That why I thought of running several bonnie++
tests in parallel, and add random seek results to simulate database
reading...

Thanks
Franck

Sorry, on the last mail I gave figures in the wrong order for random
seek: md is 15% _faster_ than hw, not the other way.