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• Need help trying to diagnose Symmetrix SAN performance issues

  - by arcain

  I am helping to benchmark hardware for a new SQL Server instance, and the volume presented to the OS for the data files is carved from a set of spindles on a Symmetrix SAN. The server has yet to have SQL Server installed, so the only activity on the box is our benchmarking. Now, our storage engineers say that this volume and it's resources are dedicated to our new server (I don't have access to see the actual SAN config) however the performance benchmarks are troubling. For example, the numbers look good until suddenly, and randomly, we see in our IO benchmarking tool wait times of 100 seconds, and disk queue lengths of 255 in perfmon. This SAN has an 8 GB cache, plus there are other applications besides ours that use the SAN. I'm wondering if (even though the spindles for our volumes should be dedicated to us) the cache may be getting hammered during the performance testing, or perhaps the spindles our volumes are on aren't really dedicated to us. We're not getting much traction from our storage engineers in helping us track down the problem, so if anybody has experience with diagnosing a problem like this and would like to share insights and troubleshooting methodologies, I'd appreciate it.

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• Can a VM perform better when only two cores instead of four cores are presented to it?

  - by arcain

  We had a VMWare VM at work with two cores allocated to it that ran a pretty heinous process in IIS. Under load the process was maxing out the CPU usage on both cores, so we asked our system engineers to present the other two cores of the physical processor to the VM. The engineer immediately said that this would not improve performance at all, but would make the VM perform worse. That statement didn't make much sense to me, and I'm wondering how what the engineer said could be true. Are there actually cases where four cores presented to a VM would cause worse performance than two cores on the same physical hardware? Let's assume an ideal situation where there's only one VM on the host server, so nothing is being shared with other OS instances. I believe the physical server had a single quad core processor, and was most likely hosting multiple VMs. I don't really know what version of ESX was running on the host, nor do I know with certainty what the physical processor config was, but from within the VM I had access to, I saw two 3.33 GHz AMD processors. In the end, I never got to test the engineer's assertion out because (while we were trying to get the VM upgraded) we were able to optimize the process and reduce it's CPU consumption, and 2) we ended up migrating to a different VM on another ESX server which had four cores presented to it.

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