Based on the comments, and the other posts that said there was no point in setting IOPS to 1 on Round Robin, I decided I was going to have to get more aggressive and test a wide variety of workloads on multiple hosts and datastores. My goal is to see if there would be any significant difference between Round Robin and PowerPath VE in a larger environment than I was testing with previously.
For Round 3 of my tests, I use 3 hosts, 9 Win2008 R2 VM’s, and 3 datastores. My hosts are HP BL460 G7 blades with HP CNA’s. All hosts are running ESXi 5 and are connected via passthrough modules to Cisco Nexus switches. FCoE is being used to the Nexus, and then FC from there to Cisco MDS’s, then to the VMAX. No Storage IO Control, DRS, or FAST is active on these hosts / LUN’s.
Here are the test VM’s, and their respective IOMeter setup:
The first test is Round Robin with the IOPS=1 setting. We’re seeing 20,673 IOPS with an average read latency of 7.69ms. Write latency is 7.5ms on this test. When we change all LUN’s back to the default of IOPS=1000, we see a significant drop in IOPS, and a 40% increase in latency. Since the bulk of my IOMeter profiles are sequential, this makes sense. EMC tests, as well as my own, show that there is little difference between IOPS=1 and IOPS=1000 when dealing with small block 100% random I/O.
When switching to PowerPath hosts, we see the IOPS increase around 6%. This is probably not statistically significant or anything, but what I did find interesting is the 15% better read latency. My guess is that PowerPath is dynamically tuning based on the workload profile from each host, where Round Robin is stuck at whatever I set as the IOPS= number.
Here’s the scorecard for Round 3:
To sum up our last round of comparisons, it was nice to see results using more hosts, datastores, and VM’s with varying I/O profiles. While this was helpful, no one can really simulate what real workloads are going to do in production, with IOMeter.
PowerPath for physical servers is a no-brainer. Based on my results, I am recommending the purchase of PowerPath VE for my VMware environment as well. In my opinion, it comes down to predictability, and peace of mind. I cannot predict what all workloads are going to look like in my environment for the future, and I am not willing to test and tune individual LUN’s with different Round Robin settings. I’d much rather leave that up to a piece of software.
Thanks for all the comments and ideas for these tests and posts.