The Fallacy of Performance or; Are you bringing your Support Agent Apples or Oranges? VM Virtualization Performance with DiskSpd.

The Fallacy of performance.

I don’t think you don’t already know this. My experience does tell me that we all group things together naturally and sometimes the performance issues we find, are really assertions made, with one piece of evidence. This performance claim is generally hard for a support agent to frame. Not that your case wont be worked, on. It will. It just it may take support teams hours or weeks to get to the truth of your statements.

I have to write this because it is so prevalent. When someone calls me and they want to open a support case, I generally try to standardize the case to some truthful statements, which I can prove, disprove, or alter.

However one such case type, that does not fit into such neat lines is the Virtualization of Performance case. Rather than describe in computer terms, let’s use American Auto Makers Ford and Chevy.

Baselines Matter

I Studied Ford and Chevy Specifications for months. I know the performance characteristics of each very well. Let’s even say I own a Chevy, and I am now looking for a second car of equal specifications. Let’s even say I own a Ford and am looking for a second ford. Then I purchase that second car, and on the way home, I find that the one car does not seem to meet the specifications of the first!

Of course I must call the car company and make the complaint that one car is not as fast as the other, or not as quick to brake, or some other specification. How about this; the air is not as cold in one as it is in the other!! This is what happens in Performance calls.

Just so you know we technically should not even entertain these types of questions. But in support we do, to some degree, because we want to help, and were not sure what your showing us yet. We don’t see the pieces for several calls.  You can’t force it, it just takes time.  This is because you are asking us to form a relationship to ideas that are not related. Two cars are not related, two computers are not related.


How about taking a cross country trip? One car crossed the country in three days. The other took 4. At this point you may be seeing my point. Trying to get down to difference in different items, of any type can be like comparing apples to oranges. What’s even worse is when one of the items is off limits. So My Ford is definitely slower, but my wife takes the other ford to work so we really can’t use that one for testing! Now what do we do?

Basic Performance assumptions

So there are some simple rules that you should apply to any performance problem:


  1. The performance should be documented and repeatable.
  2. More than one test should be run, and simple is usually more realistic.
  3. Tests should be standardized, down to a science, so that if applied to another matching scenario, you would expect similar results.
  4. Keep the time down to a short test. The longer the test, the more variables can be introduced.
  5. Do not focus on two separate car models not functioning the same, find a way to introduce a baseline into what a reasonable car will perform like. Then prove or disprove your baseline.


Now obviously, the complexity of computers can result in more rules, but if you follow these basics, you can at least find some sanity in your test results. In fact Support has an absolute need, that this happens. It is very possible, nothing is really, wrong, if we don’t get down to brass tacks.

So real world

Call into support and report when you run this command on one machine, things are fine. When you run on the other environment, things failed. This is the Disk Speed command. This is the replacement for SQLIO. I really like this tool.


  • diskspd –b8K –d30 –o4 –t8 –h –r –w25 –L –Z1G –c20G  D:\iotest.dat > DiskSpeedResults.txt


However, what is hiding in this statement, violates all the 5 rules above. This is an assertion, based on one command. Furthermore, you ran this command, in the test location,  over and over, while other VMS are also running, randomly, creating a random pattern of storage fragmentation, while the Production environment was only one once, in a very controlled situation. These commands were not run in a scientific fashion.

It literally took me a days to think of a way to baseline this situation and to  test this correctly. This is where the 5 rules came from. I think they are solid rules for support to go by. So here is how you test to make your case to Support:

User Guide and Product here

Introduce a Baseline. Anything is better then nothing

The Above diskspd command is complex and long. Come up with some simple tests and run more of them, over time. Second, test your commands, on a laptop, or desktop, with a specific Ram, Storage, and Processor Profile. Once you record all the results on the client machine, duplicate the test in the Virtual Machine. Make sure it’s the only Virtual Machine Running. Make sure nothing is running on the Host but this one VM, with specific resources.

Now below I am not giving you results. I am just giving you the commands, Along with some Instructions on how to use DSKSPD. I am also leaving you with Articles that VMware and Microsoft Hyper-V use, when asking for baseline testing. Notice, how many little requirements they have Seem familiar? There is a reason for this! We are all trying to be scientific.

Tests to establish a Baseline.

  • .\diskspd -c100M -d20 c:\test1 d:\test2
  • .\diskspd -c2G -b4K -F8 -r -o32 -W30 -d30 -Sh d:\testfile.dat
  • .\diskspd -t1 -o1 -s8k -b8k -Sh -w100 -g80 c:\test1.dat d:\test2.dat
  • .\diskspd.exe -c5G -d60 -r -w90 -t8 -o8 -b8K -h -L
  • .\diskspd.exe -c10G -d10 -r -w0 -t8 -o8 -b8K -h -L d:\testfile.dat
  • .\scriptname.ps1
  • Same as above- second location
  • .\Diskspd -b8k -d30 -Sh  -o8 -t8  -w20 -c2G d:\iotest.dat


This list will generate about 15 unique results. Any of these will run on a laptop or a server. Just make sure you read the text character decoder sheet available with the product.

So the instructions are very simple. The specs on the Hyper-V or VMware VM, must be the same as the laptop. My laptop has 16GB of ram, and 8 Processors.

The VM must be the only one running, and the OS should be a fresh Install. Now if the results of testing are in the ballpark of your comparison Client, then you are not having a performance issue.

The moral of the story is test from different perspectives, and use the Scientific method, as much as you are able to.

I hope this is helpful in your troubleshooting.


A few other Details

Here is a way to manually pre-create the files if disired

  • fsutil file createnew d:\iotest.dat 20000000
  • fsutil file createnew d:\iotest.dat 2000000000
  • fsutil file createnew d:\iotest.dat 20000000000

Here is all of the best articles on storage, and IO online right now. I was surprised that so many of  Storage Performance Needs are all in one place.

This could be an important point. If you came to this site because your numbers are not matching reality, your Monitoring tools, may not be collecting the right perfmon numbers, then you may need the Hyper-V performance script to use to see your actual VM numbers. try using this tool Run this tool on the host, while using diskspd on your VM.

DO not run more then one instance of Diskspd at once!! This will invalidate your tests!


Finally, as promised, here is how  VMware or Microsoft  handle these issues:




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