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So I learned that since Red Hat 6 it is possible to choose between 3 algorithms for loadbalancing between different paths to my FC-SAN, the default dumb round-robin - and new - queue-length and service-time.

I have been searching for a long time now for a performance-comparison for different values for path_selector in a multibus-configuration, but I was not able to find any information based on real experience. I did not even find an assumption of what performs better in which situations.

Does anyone have done some real performance-testing on this question? Or information about resources I was not able to find?

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I have a few servers that use queue-length and they haven't had any problems. In general, though, there's not much to optimize in terms of multipath. We still use basic round-robin for the vast majority of our servers -- it's a well-known setup with little unknown risk, and the performance upside for queue or service time based multipathing, at least in my shop, is small.

Do you have a specific problem you're trying to address? Or are you just making sure you're set up as well as possible?

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In the meantime, I did some tests, the differences are in fact marginal:

                        round-robin queue-length    service-time
Write char KB/sec       872         884             876
Write block KB/sec      327345      331218          330533
Rewrite KB/sec          144384      144102          141393
Read char KB/sec        3714        3900            3485
Read block KB/sec       363308      349765          360518
Random Seeks count/sec  19          18  

I used bonnie++ on an ext4-filesystem.

  • The difference may not even be attributable to the multipath setup. – Basil Feb 26 '15 at 17:55

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