I've been promising I should re-visit once more the disk bound sysbench tests I ran on Galera. In December I finally had some lab time to do that. If you remember what troubled me then it was that in all my other Galera benchmarks performance with Galera was equal or much better compared to performance on a single MySQL node. (And this is very unusual wrt high availability solutions, usually they come with a performance penalty. This is why Galera is so great.) However, on the tests with a disk bound workload, there was performance degradation, and what was even more troubling was the performance seemed to decrease more when adding more write masters.
In these tests I was able to understand the performance decrease and it had nothing to do with Galera and not even InnoDB. It's a defect in my lab setup: all nodes kept their data on a partition mounted from an EMC SAN device - the same device for all nodes. Hence, when directing work to more nodes, and the workload is bottlenecked by disk access, naturally performance would decrease rather than increase. Unfortunately I don't currently have servers available (but will have sometime during this year) where I could re-run this same test with local disks.
As part of this lab session I also investigated the effect varying the number of Galera slave applier threads, which I will report on in the remainder of this post. Of course, the results are a bit obscure now due to the problematic lab setup wrt SAN, but I'll make some observations nevertheless.
While the previous tests were run on MySQL 5.1, this test was run on MySQL 5.5 and I will make some observations there too.