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

[Kaul]

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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