How cool is vscsiStats? Part-I

It has been few days since I published a white paper on the performance characterization of SQL server-based vCenter database. Few people have asked me questions about the vscsiStats graphs in the appendix of the paper. Instead of answering the questions individually I decided to blog here for the benefit of all the readers.

As mentioned in the paper, I observed this rather unusual (yes, I say unusual because I didn’t expect performance of virtual I/O stack to be better than that of native) during some of the experiments. Using the vCenter application to reproduce this behavior was rather complex and involved too many variables. Hence I decided to use a simple I/O benchmark, which most of you are familiar – IOmeter ( But to reproduce the issue, I needed to use the exact same I/O load as that produced by the stored procedures of the vCenter database. To create a custom workload profile in IOmeter I was required to configure outstanding IOs, I/O request size, read percentage and percentage of randomness (at least). Question was – how to get these parameters from the workload?

vscsiStats provided the answer. Scott Drummonds (during his VMware days) wrote a great blog on vscsiStats. I highly encourage the readers to go through the article and understand the basics of vscsiStats (if you are not already familiar with the tool).  This will help you appreciate the content of this multi-part blog series. Instead of dwelling on the details of vscsiStats, I will illustrate the usefulness of vscsiStats here.

First, a quick description of a sample histogram output from vscsiStats. All the histograms have similar format and should be straightforward to understand.

If you are curious about this tool and want to learn more, check out these technical literatures:

  1. Storage Workload Characterization and Consolidation in Virtualized Enviornments” – Ajay Gulati, Chethan Kumar, and Irfan Ahmad presented at VPACT 09 (Yes, I was one of the authors)
  2. vscsiStats: Fast and Easy Disk Workload Characterization on VMware ESX Server” – Presentation by Irfan Ahmad at VMworld 2007 (excellent presentation by one of the creators of this tool)

Up next: The histograms I collected …

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