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Re: [dm-devel] default value for rr_min_io too high?



Christophe Varoqui wrote:
On mer, 2006-01-18 at 23:29 +0100, Christophe Varoqui wrote:
 > On mer, 2006-01-18 at 16:41 -0500, David Wysochanski wrote:
 > > I'm wondering where the value of 1000 came from, and
 > > whether that's really a good default.
 > >
 > > Some preliminary tests I've run with iSCSI seem to indicate
 > > something lower (say 100) might be a better default, but
 > > perhaps others have a differing opinion.  I searched the
 > > list but couldn't find any discussion on it.
 > >
 > I'm not really focused on performance, but this seems to be an
 > io-pattern dependant choice.
 >
 > Higher values may help the elevators, (right ?) thus help the seeky
 > workloads. Lower values may certainly benefit from lower values to
 > really get the paths summed bandwidth.
 >
 > Anyway, I can not back this with numbers. Any value will be fine with me
 > as a default, and I highlight that now you can also set per device
 > defaults like rr_min_io in hwtable.c
 >
Replying to myself,

I finally got the chance to challenge my sayings, and I'm proven badly
wrong :/

On a StorageWorks EVA110 FC array, 2 active 2Gb/s paths to 2 2Gb/s
target ports. 1 streaming read (sg_dd dio=1 if=/dev/mapper/mpath0
of=/dev/null bs=1M count=100k) :

rr_min_io = 1000 => aggregated throughput = 120 Mo/s
rr_min_io =  100 => aggregated throughput = 130 Mo/s
rr_min_io =   50 => aggregated throughput = 200 Mo/s
rr_min_io =   20 => aggregated throughput = 260 Mo/s
rr_min_io =   10 => aggregated throughput = 300 Mo/s


What I seemed to see what the larger the I/O size the lower
I needed to go with rr_min_io to get best throughput.  Did
you run it with a smaller block size, say 4k?

I will try to get some more definitive #'s and post.


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