> 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@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@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@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.
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