[dm-devel] DM-Multipath limiting IO to 127.000 MB/Sec

[Ali Poursamadi]

Hi Gurus,

I'm experiencing an issue with DM-Multipath on Centos 6.5.
DD reads 2.5 times faster when reading from a device directly compared to
Multipath-ed device.
I'm using device-mapper-multipath-0.4.9-72.el6_5.2.x86_64.
Apparently Multipath is limiting throughput to 127.000 MB/s even though
the underlying device is capable of more throughput which to me it looks
like a software imposed limit.

[root at JS2 ~]# multipath -l mpathaf
mpathaf (--------) dm-89 Rorke,G4S-16L-4F8
size=7.3T features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=0 status=active
  |- 0:0:0:1  sdb  8:16   active undef running
  |- 0:0:10:2 sdbb 67:80  active undef running
  |- 1:0:11:1 sddl 71:48  active undef running
  `- 1:0:12:2 sddp 71:112 active undef running

[root at JS2 ~]#dd if=/dev/sddl of=/dev/null bs=128k count=32000 skip=14000
32000+0 records in
32000+0 records out
4194304000 bytes (4.2 GB) copied, 11.8166 s, 355 MB/s

[root at JS2 ~]# dd if=/dev/dm-89 of=/dev/null bs=128k count=32000 skip=84000
# skipping 2GB array cache data
32000+0 records in
32000+0 records out
4194304000 bytes (4.2 GB) copied, 31.4959 s, 133 MB/s


This is on a machine connected over 2x8Gb/s links to a fiber switch and
then to an HDX4 disk array, exporting the volume to 2 of it's 4 8Gbps fiber
ports .

Multipath.conf looks like this :

defaults {
        user_friendly_names yes
}

devices {

        device { # normal HDX4 Disk arrays
                vendor "Rorke"
                product "G4S-16L-4F8"
                path_grouping_policy multibus
                }

}

blacklist {
#blacklisting almost everything
        devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
        devnode "^hd[a-z][0-9]*"
}


Any help/pointer/information is highly appreciated.

Thanks.
Ali Poursamadi
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