[dm-devel] [Multipath] Round-robin performance limit

[Adam Chasen]

Lowering rr_min_io provides marginal improvement. I see 6MB/s
improvement at an rr_min_io of 3 vs 100. I played around with it
before all the way down to 1. People seems to settle on 3. Still, I am
not seeing the bandwidth I assume it should be (4 aggregated links).

Some additional information. If I attempt to pull from my two
multipath devices simultaneously (different LUNs, but same iSCSI
connections) then I can pull additional data (50MB/s vs 27-30MB/s
from each link).

Adam

This is a response to a direct email I sent to someone who had a
similar issue on this list a while back:
Date: Sat, 30 Apr 2011 00:13:20 +0200
From: Bart Coninckx <bart.coninckx at telenet.be>
Hi Adam,

I believe setting rr_min_io to 3 in stead of 100 improved things
significantly.
What is still an unexplainable issue though is dd-ing to the multipath
device (very slow) while reading from it is very fast. Doing the same
piped over SSH to the original devices on the iSCSI server was OK, so it
seems like either an iSCSI or still a multipath issue.

But I definitely remember that lowering rr_min_io helped quite a bit.
I think the paths are switched faster in this way resulting into more speed.

Good luck,

b.


On Mon, May 2, 2011 at 3:25 AM, Pasi Kärkkäinen <pasik at iki.fi> wrote:
> On Thu, Apr 28, 2011 at 11:55:55AM -0400, Adam Chasen wrote:
>>
>> [root at zed ~]# multipath -ll
>> 3600c0ff000111346d473554d01000000 dm-3 DotHill,DH3000
>> size=1.1T features='0' hwhandler='0' wp=rw
>> `-+- policy='round-robin 0' prio=1 status=active
>>   |- 88:0:0:0 sdd 8:48  active ready  running
>>   |- 86:0:0:0 sdc 8:32  active ready  running
>>   |- 89:0:0:0 sdg 8:96  active ready  running
>>   `- 87:0:0:0 sdf 8:80  active ready  running
>> 3600c0ff00011148af973554d01000000 dm-2 DotHill,DH3000
>> size=1.1T features='0' hwhandler='0' wp=rw
>> `-+- policy='round-robin 0' prio=1 status=active
>>   |- 89:0:0:1 sdk 8:160 active ready  running
>>   |- 88:0:0:1 sdi 8:128 active ready  running
>>   |- 86:0:0:1 sdh 8:112 active ready  running
>>   `- 87:0:0:1 sdl 8:176 active ready  running
>>
>> /etc/multipath.conf
>> defaults {
>>         path_grouping_policy    multibus
>>         rr_min_io 100
>> }
>
> Did you try a lower value for rr_min_io ?
>
> -- Pasi
>
>>
>> multipath-tools v0.4.9 (05/33, 2016)
>> 2.6.35.11-2-fl.smp.gcc4.4.x86_64
>>
>> Thanks,
>> Adam
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