[dm-devel] Substantial performance difference when reading/writing to device-mapper vs. the individual device

[Kaul]

Reply to self:

Could it be explained by the difference in max_segments between the
different devices and the dm device?
Sounds like https://bugzilla.redhat.com/show_bug.cgi?id=755046 which is
supposed to be fixed in 6.4, I reckon:



3514f0c5615a00003 dm-3 XtremIO,XtremApp

size=1.0T features='0' hwhandler='0' wp=rw

`-+- policy='queue-length 0' prio=1 status=active

  |- 0:0:2:2 sdi  8:128  active ready running

  |- 0:0:3:2 sdl  8:176  active ready running

  |- 0:0:1:2 sdf  8:80   active ready running

  |- 0:0:0:2 sdc  8:32   active ready running

  |- 1:0:0:2 sds  65:32  active ready running

  |- 1:0:3:2 sdab 65:176 active ready running

  |- 1:0:2:2 sdy  65:128 active ready running

  `- 1:0:1:2 sdv  65:80  active ready running



[root at lg545 ~]# cat /sys/class/block/dm-3/queue/max_segments

128

[root at lg545 ~]# cat /sys/class/block/sdi/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sdl/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sdf/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sdc/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sds/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sdab/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sdy/queue/max_segments

1024

[root at lg545 ~]# cat /sys/class/block/sdv/queue/max_segments

1024


On Mon, Jul 22, 2013 at 2:47 PM, Kaul <mykaul at gmail.com> wrote:

> We are seeing a substantial difference in performance when we perform a
> read/write to /dev/mapper/... vs. the specific device (/dev/sdXX)
> What can we do to further isolate the issue?
>
> We are using CentOS 6.4, with all updates, 2 CPUs, 4 FC ports:
> Here's a table comparing the results:
>
> # of LUNs
>
> # of Paths per device
>
> Native Multipath Device
>
> IO Pattern
>
> IOPS
>
> Latency Micro
>
> BW KBps
>
> 4
>
> 16
>
> No
>
> 100% Read
>
> 605,661.4
>
> 3,381
>
> 2,420,736
>
> 4
>
> 16
>
> No
>
> 100% Write
>
> 477,515.1
>
> 4,288
>
> 1,908,736
>
> 8
>
> 16
>
> No
>
> 100% Read
>
> 663,339.4
>
> 6,174
>
> 2,650,112
>
> 8
>
> 16
>
> No
>
> 100% Write
>
> 536,936.9
>
> 7,628
>
> 2,146,304
>
> 4
>
> 16
>
> Yes
>
> 100% Read
>
> 456,108.9
>
> 1,122
>
> 1,824,256
>
> 4
>
> 16
>
> Yes
>
> 100% Write
>
> 371,665.8
>
> 1,377
>
> 1,486,336
>
> 8
>
> 16
>
> Yes
>
> 100% Read
>
> 519,450.2
>
> 1,971
>
> 2,077,696
>
> 8
>
> 16
>
> Yes
>
> 100% Write
>
> 448,840.4
>
> 2,281
>
> 1,795,072
>
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