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

[Ali Poursamadi]

Thanks ShivaKrishna,

max_sectors_kb , max_segments, max_segment_size is identical on both sdx-s
and the relevan dm-x device and are as follows :
max_sectors_kb : 512
max_segments : 1024
max_segment_size :  65536

I did the iostat tests, turns out that average Request Size show close
readings ( 244,251 for dm-x, sdx respectively ) but the Average Queue Size
is close to 1.9 for sdx vs 1.1 for dm-x and also the device utilization
(%util) is ~99% on sd-x vs ~60% on dm-xx. It seems that dm multipath is
under-utilizing the underlaying devices.
is there a way to change this behavior ?

-Ali Poursamadi





On Tue, Sep 23, 2014 at 7:45 AM, Merla, ShivaKrishna <
ShivaKrishna.Merla at netapp.com> wrote:

> > 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]*"
> > }
>
> Check the request queue parameters for both dm and sd devices. Parameters
> that could affect this are
>
> #cat /sys/block/<dm-x>/queue/max_sectors_kb
> #cat /sys/block/<dm-x>/queue/max_segments
> #cat /sys/block/<dm-x>/queue/max_segment_size
>
> #cat /sys/block/<sdx>/queue/max_sectors_kb
> #cat /sys/block/<sdx>/queue/max_segments
> #cat /sys/block/<sdx>/queue/max_segment_size
>
> You can monitor the average request size(avgrq-sz) on both dm and sd
> device using
> #iostat –xm –d /dev/dm-x /dev/sdx –t 2
>
> Compare this with the request size while running I/O only sd device
> directly. There was a patch to fix
> block stacking limits on dm devices upstream.
>
>
>
>
>
> --
> dm-devel mailing list
> dm-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel




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